Chapter XIII

Mutual Affinities of Organic Beings:

Morphology: Embryology: Rudimentary Organs

Classification, groups subordinate to groups -- Natural system -- Rules and

difficulties in classification, explained on the theory of descent with

modification -- Classification of varieties -- Descent always used in

classification -- Analogical or adaptive characters -- Affinities, general,

complex and radiating -- Extinction separates and defines groups --

Morphology, between members of the same class, between parts of the same

individual -- Embryology, laws of, explained by variations not supervening

at an early age, and being inherited at a corresponding age -- Rudimentary

Organs; their origin explained -- Summary.

From the first dawn of life, all organic beings are found to resemble each

other in descending degrees, so that they can be classed in groups under

groups. This classification is evidently not arbitrary like the grouping

of the stars in constellations. The existence of groups would have been of

simple signification, if one group had been exclusively fitted to inhabit

the land, and another the water; one to feed on flesh, another on vegetable

matter, and so on; but the case is widely different in nature; for it is

notorious how commonly members of even the same subgroup have different

habits. In our second and fourth chapters, on Variation and on Natural

Selection, I have attempted to show that it is the widely ranging, the much

diffused and common, that is the dominant species belonging to the larger

genera, which vary most. The varieties, or incipient species, thus

produced ultimately become converted, as I believe, into new and distinct

species; and these, on the principle of inheritance, tend to produce other

new and dominant species. Consequently the groups which are now large, and

which generally include many dominant species, tend to go on increasing

indefinitely in size. I further attempted to show that from the varying

descendants of each species trying to occupy as many and as different

places as possible in the economy of nature, there is a constant tendency

in their characters to diverge. This conclusion was supported by looking

at the great diversity of the forms of life which, in any small area, come

into the closest competition, and by looking to certain facts in

naturalisation.

I attempted also to show that there is a constant tendency in the forms

which are increasing in number and diverging in character, to supplant and

exterminate the less divergent, the less improved, and preceding forms. I

request the reader to turn to the diagram illustrating the action, as

formerly explained, of these several principles; and he will see that the

inevitable result is that the modified descendants proceeding from one

progenitor become broken up into groups subordinate to groups. In the

diagram each letter on the uppermost line may represent a genus including

several species; and all the genera on this line form together one class,

for all have descended from one ancient but unseen parent, and,

consequently, have inherited something in common. But the three genera on

the left hand have, on this same principle, much in common, and form a

sub-family, distinct from that including the next two genera on the right

hand, which diverged from a common parent at the fifth stage of descent.

These five genera have also much, though less, in common; and they form a

family distinct from that including the three genera still further to the

right hand, which diverged at a still earlier period. And all these

genera, descended from (A), form an order distinct from the genera

descended from (I). So that we here have many species descended from a

single progenitor grouped into genera; and the genera are included in, or

subordinate to, sub-families, families, and orders, all united into one

class. Thus, the grand fact in natural history of the subordination of

group under group, which, from its familiarity, does not always

sufficiently strike us, is in my judgment fully explained.

Naturalists try to arrange the species, genera, and families in each class,

on what is called the Natural System. But what is meant by this system?

Some authors look at it merely as a scheme for arranging together those

living objects which are most alike, and for separating those which are

most unlike; or as an artificial means for enunciating, as briefly as

possible, general propositions,--that is, by one sentence to give the

characters common, for instance, to all mammals, by another those common to

all carnivora, by another those common to the dog-genus, and then by adding

a single sentence, a full description is given of each kind of dog. The

ingenuity and utility of this system are indisputable. But many

naturalists think that something more is meant by the Natural System; they

believe that it reveals the plan of the Creator; but unless it be specified

whether order in time or space, or what else is meant by the plan of the

Creator, it seems to me that nothing is thus added to our knowledge. Such

expressions as that famous one of Linnaeus, and which we often meet with in

a more or less concealed form, that the characters do not make the genus,

but that the genus gives the characters, seem to imply that something more

is included in our classification, than mere resemblance. I believe that

something more is included; and that propinquity of descent,--the only

known cause of the similarity of organic beings,--is the bond, hidden as it

is by various degrees of modification, which is partially revealed to us by

our classifications.

Let us now consider the rules followed in classification, and the

difficulties which are encountered on the view that classification either

gives some unknown plan of creation, or is simply a scheme for enunciating

general propositions and of placing together the forms most like each

other. It might have been thought (and was in ancient times thought) that

those parts of the structure which determined the habits of life, and the

general place of each being in the economy of nature, would be of very high

importance in classification. Nothing can be more false. No one regards

the external similarity of a mouse to a shrew, of a dugong to a whale, of a

whale to a fish, as of any importance. These resemblances, though so

intimately connected with the whole life of the being, are ranked as merely

'adaptive or analogical characters;' but to the consideration of these

resemblances we shall have to recur. It may even be given as a general

rule, that the less any part of the organisation is concerned with special

habits, the more important it becomes for classification. As an instance:

Owen, in speaking of the dugong, says, 'The generative organs being those

which are most remotely related to the habits and food of an animal, I have

always regarded as affording very clear indications of its true affinities.

We are least likely in the modifications of these organs to mistake a

merely adaptive for an essential character.' So with plants, how

remarkable it is that the organs of vegetation, on which their whole life

depends, are of little signification, excepting in the first main

divisions; whereas the organs of reproduction, with their product the seed,

are of paramount importance!

We must not, therefore, in classifying, trust to resemblances in parts of

the organisation, however important they may be for the welfare of the

being in relation to the outer world. Perhaps from this cause it has

partly arisen, that almost all naturalists lay the greatest stress on

resemblances in organs of high vital or physiological importance. No doubt

this view of the classificatory importance of organs which are important is

generally, but by no means always, true. But their importance for

classification, I believe, depends on their greater constancy throughout

large groups of species; and this constancy depends on such organs having

generally been subjected to less change in the adaptation of the species to

their conditions of life. That the mere physiological importance of an

organ does not determine its classificatory value, is almost shown by the

one fact, that in allied groups, in which the same organ, as we have every

reason to suppose, has nearly the same physiological value, its

classificatory value is widely different. No naturalist can have worked at

any group without being struck with this fact; and it has been most fully

acknowledged in the writings of almost every author. It will suffice to

quote the highest authority, Robert Brown, who in speaking of certain

organs in the Proteaceae, says their generic importance, 'like that of all

their parts, not only in this but, as I apprehend, in every natural family,

is very unequal, and in some cases seems to be entirely lost.' Again in

another work he says, the genera of the Connaraceae 'differ in having one

or more ovaria, in the existence or absence of albumen, in the imbricate or

valvular aestivation. Any one of these characters singly is frequently of

more than generic importance, though here even when all taken together they

appear insufficient to separate Cnestis from Connarus.' To give an example

amongst insects, in one great division of the Hymenoptera, the antennae, as

Westwood has remarked, are most constant in structure; in another division

they differ much, and the differences are of quite subordinate value in

classification; yet no one probably will say that the antennae in these two

divisions of the same order are of unequal physiological importance. Any

number of instances could be given of the varying importance for

classification of the same important organ within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are of high

physiological or vital importance; yet, undoubtedly, organs in this

condition are often of high value in classification. No one will dispute

that the rudimentary teeth in the upper jaws of young ruminants, and

certain rudimentary bones of the leg, are highly serviceable in exhibiting

the close affinity between Ruminants and Pachyderms. Robert Brown has

strongly insisted on the fact that the rudimentary florets are of the

highest importance in the classification of the Grasses.

Numerous instances could be given of characters derived from parts which

must be considered of very trifling physiological importance, but which are

universally admitted as highly serviceable in the definition of whole

groups. For instance, whether or not there is an open passage from the

nostrils to the mouth, the only character, according to Owen, which

absolutely distinguishes fishes and reptiles--the inflection of the angle

of the jaws in Marsupials--the manner in which the wings of insects are

folded--mere colour in certain Algae--mere pubescence on parts of the

flower in grasses--the nature of the dermal covering, as hair or feathers,

in the Vertebrata. If the Ornithorhynchus had been covered with feathers

instead of hair, this external and trifling character would, I think, have

been considered by naturalists as important an aid in determining the

degree of affinity of this strange creature to birds and reptiles, as an

approach in structure in any one internal and important organ.

The importance, for classification, of trifling characters, mainly depends

on their being correlated with several other characters of more or less

importance. The value indeed of an aggregate of characters is very evident

in natural history. Hence, as has often been remarked, a species may

depart from its allies in several characters, both of high physiological

importance and of almost universal prevalence, and yet leave us in no doubt

where it should be ranked. Hence, also, it has been found, that a

classification founded on any single character, however important that may

be, has always failed; for no part of the organisation is universally

constant. The importance of an aggregate of characters, even when none are

important, alone explains, I think, that saying of Linnaeus, that the

characters do not give the genus, but the genus gives the characters; for

this saying seems founded on an appreciation of many trifling points of

resemblance, too slight to be defined. Certain plants, belonging to the

Malpighiaceae, bear perfect and degraded flowers; in the latter, as A. de

Jussieu has remarked, 'the greater number of the characters proper to the

species, to the genus, to the family, to the class, disappear, and thus

laugh at our classification.' But when Aspicarpa produced in France,

during several years, only degraded flowers, departing so wonderfully in a

number of the most important points of structure from the proper type of

the order, yet M. Richard sagaciously saw, as Jussieu observes, that this

genus should still be retained amongst the Malpighiaceae. This case seems

to me well to illustrate the spirit with which our classifications are

sometimes necessarily founded.

Practically when naturalists are at work, they do not trouble themselves

about the physiological value of the characters which they use in defining

a group, or in allocating any particular species. If they find a character

nearly uniform, and common to a great number of forms, and not common to

others, they use it as one of high value; if common to some lesser number,

they use it as of subordinate value. This principle has been broadly

confessed by some naturalists to be the true one; and by none more clearly

than by that excellent botanist, Aug. St. Hilaire. If certain characters

are always found correlated with others, though no apparent bond of

connexion can be discovered between them, especial value is set on them.

As in most groups of animals, important organs, such as those for

propelling the blood, or for aerating it, or those for propagating the

race, are found nearly uniform, they are considered as highly serviceable

in classification; but in some groups of animals all these, the most

important vital organs, are found to offer characters of quite subordinate

value.

We can see why characters derived from the embryo should be of equal

importance with those derived from the adult, for our classifications of

course include all ages of each species. But it is by no means obvious, on

the ordinary view, why the structure of the embryo should be more important

for this purpose than that of the adult, which alone plays its full part in

the economy of nature. Yet it has been strongly urged by those great

naturalists, Milne Edwards and Agassiz, that embryonic characters are the

most important of any in the classification of animals; and this doctrine

has very generally been admitted as true. The same fact holds good with

flowering plants, of which the two main divisions have been founded on

characters derived from the embryo,--on the number and position of the

embryonic leaves or cotyledons, and on the mode of development of the

plumule and radicle. In our discussion on embryology, we shall see why

such characters are so valuable, on the view of classification tacitly

including the idea of descent.

Our classifications are often plainly influenced by chains of affinities.

Nothing can be easier than to define a number of characters common to all

birds; but in the case of crustaceans, such definition has hitherto been

found impossible. There are crustaceans at the opposite ends of the

series, which have hardly a character in common; yet the species at both

ends, from being plainly allied to others, and these to others, and so

onwards, can be recognised as unequivocally belonging to this, and to no

other class of the Articulata.

Geographical distribution has often been used, though perhaps not quite

logically, in classification, more especially in very large groups of

closely allied forms. Temminck insists on the utility or even necessity of

this practice in certain groups of birds; and it has been followed by

several entomologists and botanists.

Finally, with respect to the comparative value of the various groups of

species, such as orders, sub-orders, families, sub-families, and genera,

they seem to be, at least at present, almost arbitrary. Several of the

best botanists, such as Mr. Bentham and others, have strongly insisted on

their arbitrary value. Instances could be given amongst plants and

insects, of a group of forms, first ranked by practised naturalists as only

a genus, and then raised to the rank of a sub-family or family; and this

has been done, not because further research has detected important

structural differences, at first overlooked, but because numerous allied

species, with slightly different grades of difference, have been

subsequently discovered.

All the foregoing rules and aids and difficulties in classification are

explained, if I do not greatly deceive myself, on the view that the natural

system is founded on descent with modification; that the characters which

naturalists consider as showing true affinity between any two or more

species, are those which have been inherited from a common parent, and, in

so far, all true classification is genealogical; that community of descent

is the hidden bond which naturalists have been unconsciously seeking, and

not some unknown plan of creation, or the enunciation of general

propositions, and the mere putting together and separating objects more or

less alike.

But I must explain my meaning more fully. I believe that the arrangement

of the groups within each class, in due subordination and relation to the

other groups, must be strictly genealogical in order to be natural; but

that the amount of difference in the several branches or groups, though

allied in the same degree in blood to their common progenitor, may differ

greatly, being due to the different degrees of modification which they have

undergone; and this is expressed by the forms being ranked under different

genera, families, sections, or orders. The reader will best understand

what is meant, if he will take the trouble of referring to the diagram in

the fourth chapter. We will suppose the letters A to L to represent allied

genera, which lived during the Silurian epoch, and these have descended

from a species which existed at an unknown anterior period. Species of

three of these genera (A, F, and I) have transmitted modified descendants

to the present day, represented by the fifteen genera (a14 to z14) on the

uppermost horizontal line. Now all these modified descendants from a

single species, are represented as related in blood or descent to the same

degree; they may metaphorically be called cousins to the same millionth

degree; yet they differ widely and in different degrees from each other.

The forms descended from A, now broken up into two or three families,

constitute a distinct order from those descended from I, also broken up

into two families. Nor can the existing species, descended from A, be

ranked in the same genus with the parent A; or those from I, with the

parent I. But the existing genus F14 may be supposed to have been but

slightly modified; and it will then rank with the parent-genus F; just as

some few still living organic beings belong to Silurian genera. So that

the amount or value of the differences between organic beings all related

to each other in the same degree in blood, has come to be widely different.

Nevertheless their genealogical arrangement remains strictly true, not only

at the present time, but at each successive period of descent. All the

modified descendants from A will have inherited something in common from

their common parent, as will all the descendants from I; so will it be with

each subordinate branch of descendants, at each successive period. If,

however, we choose to suppose that any of the descendants of A or of I have

been so much modified as to have more or less completely lost traces of

their parentage, in this case, their places in a natural classification

will have been more or less completely lost,--as sometimes seems to have

occurred with existing organisms. All the descendants of the genus F,

along its whole line of descent, are supposed to have been but little

modified, and they yet form a single genus. But this genus, though much

isolated, will still occupy its proper intermediate position; for F

originally was intermediate in character between A and I, and the several

genera descended from these two genera will have inherited to a certain

extent their characters. This natural arrangement is shown, as far as is

possible on paper, in the diagram, but in much too simple a manner. If a

branching diagram had not been used, and only the names of the groups had

been written in a linear series, it would have been still less possible to

have given a natural arrangement; and it is notoriously not possible to

represent in a series, on a flat surface, the affinities which we discover

in nature amongst the beings of the same group. Thus, on the view which I

hold, the natural system is genealogical in its arrangement, like a

pedigree; but the degrees of modification which the different groups have

undergone, have to be expressed by ranking them under different so-called

genera, sub-families, families, sections, orders, and classes.

It may be worth while to illustrate this view of classification, by taking

the case of languages. If we possessed a perfect pedigree of mankind, a

genealogical arrangement of the races of man would afford the best

classification of the various languages now spoken throughout the world;

and if all extinct languages, and all intermediate and slowly changing

dialects, had to be included, such an arrangement would, I think, be the

only possible one. Yet it might be that some very ancient language had

altered little, and had given rise to few new languages, whilst others

(owing to the spreading and subsequent isolation and states of civilisation

of the several races, descended from a common race) had altered much, and

had given rise to many new languages and dialects. The various degrees of

difference in the languages from the same stock, would have to be expressed

by groups subordinate to groups; but the proper or even only possible

arrangement would still be genealogical; and this would be strictly

natural, as it would connect together all languages, extinct and modern, by

the closest affinities, and would give the filiation and origin of each

tongue.

In confirmation of this view, let us glance at the classification of

varieties, which are believed or known to have descended from one species.

These are grouped under species, with sub-varieties under varieties; and

with our domestic productions, several other grades of difference are

requisite, as we have seen with pigeons. The origin of the existence of

groups subordinate to groups, is the same with varieties as with species,

namely, closeness of descent with various degrees of modification. Nearly

the same rules are followed in classifying varieties, as with species.

Authors have insisted on the necessity of classing varieties on a natural

instead of an artificial system; we are cautioned, for instance, not to

class two varieties of the pine-apple together, merely because their fruit,

though the most important part, happens to be nearly identical; no one puts

the swedish and common turnips together, though the esculent and thickened

stems are so similar. Whatever part is found to be most constant, is used

in classing varieties: thus the great agriculturist Marshall says the

horns are very useful for this purpose with cattle, because they are less

variable than the shape or colour of the body, &c.; whereas with sheep the

horns are much less serviceable, because less constant. In classing

varieties, I apprehend if we had a real pedigree, a genealogical

classification would be universally preferred; and it has been attempted by

some authors. For we might feel sure, whether there had been more or less

modification, the principle of inheritance would keep the forms together

which were allied in the greatest number of points. In tumbler pigeons,

though some sub-varieties differ from the others in the important character

of having a longer beak, yet all are kept together from having the common

habit of tumbling; but the short-faced breed has nearly or quite lost this

habit; nevertheless, without any reasoning or thinking on the subject,

these tumblers are kept in the same group, because allied in blood and

alike in some other respects. If it could be proved that the Hottentot had

descended from the Negro, I think he would be classed under the Negro

group, however much he might differ in colour and other important

characters from negroes.

With species in a state of nature, every naturalist has in fact brought

descent into his classification; for he includes in his lowest grade, or

that of a species, the two sexes; and how enormously these sometimes differ

in the most important characters, is known to every naturalist: scarcely a

single fact can be predicated in common of the males and hermaphrodites of

certain cirripedes, when adult, and yet no one dreams of separating them.

The naturalist includes as one species the several larval stages of the

same individual, however much they may differ from each other and from the

adult; as he likewise includes the so-called alternate generations of

Steenstrup, which can only in a technical sense be considered as the same

individual. He includes monsters; he includes varieties, not solely

because they closely resemble the parent-form, but because they are

descended from it. He who believes that the cowslip is descended from the

primrose, or conversely, ranks them together as a single species, and gives

a single definition. As soon as three Orchidean forms (Monochanthus,

Myanthus, and Catasetum), which had previously been ranked as three

distinct genera, were known to be sometimes produced on the same spike,

they were immediately included as a single species. But it may be asked,

what ought we to do, if it could be proved that one species of kangaroo had

been produced, by a long course of modification, from a bear? Ought we to

rank this one species with bears, and what should we do with the other

species? The supposition is of course preposterous; and I might answer by

the argumentum ad hominem, and ask what should be done if a perfect

kangaroo were seen to come out of the womb of a bear? According to all

analogy, it would be ranked with bears; but then assuredly all the other

species of the kangaroo family would have to be classed under the bear

genus. The whole case is preposterous; for where there has been close

descent in common, there will certainly be close resemblance or affinity.

As descent has universally been used in classing together the individuals

of the same species, though the males and females and larvae are sometimes

extremely different; and as it has been used in classing varieties which

have undergone a certain, and sometimes a considerable amount of

modification, may not this same element of descent have been unconsciously

used in grouping species under genera, and genera under higher groups,

though in these cases the modification has been greater in degree, and has

taken a longer time to complete? I believe it has thus been unconsciously

used; and only thus can I understand the several rules and guides which

have been followed by our best systematists. We have no written pedigrees;

we have to make out community of descent by resemblances of any kind.

Therefore we choose those characters which, as far as we can judge, are the

least likely to have been modified in relation to the conditions of life to

which each species has been recently exposed. Rudimentary structures on

this view are as good as, or even sometimes better than, other parts of the

organisation. We care not how trifling a character may be--let it be the

mere inflection of the angle of the jaw, the manner in which an insect's

wing is folded, whether the skin be covered by hair or feathers--if it

prevail throughout many and different species, especially those having very

different habits of life, it assumes high value; for we can account for its

presence in so many forms with such different habits, only by its

inheritance from a common parent. We may err in this respect in regard to

single points of structure, but when several characters, let them be ever

so trifling, occur together throughout a large group of beings having

different habits, we may feel almost sure, on the theory of descent, that

these characters have been inherited from a common ancestor. And we know

that such correlated or aggregated characters have especial value in

classification.

We can understand why a species or a group of species may depart, in

several of its most important characteristics, from its allies, and yet be

safely classed with them. This may be safely done, and is often done, as

long as a sufficient number of characters, let them be ever so unimportant,

betrays the hidden bond of community of descent. Let two forms have not a

single character in common, yet if these extreme forms are connected

together by a chain of intermediate groups, we may at once infer their

community of descent, and we put them all into the same class. As we find

organs of high physiological importance--those which serve to preserve life

under the most diverse conditions of existence--are generally the most

constant, we attach especial value to them; but if these same organs, in

another group or section of a group, are found to differ much, we at once

value them less in our classification. We shall hereafter, I think,

clearly see why embryological characters are of such high classificatory

importance. Geographical distribution may sometimes be brought usefully

into play in classing large and widely-distributed genera, because all the

species of the same genus, inhabiting any distinct and isolated region,

have in all probability descended from the same parents.

We can understand, on these views, the very important distinction between

real affinities and analogical or adaptive resemblances. Lamarck first

called attention to this distinction, and he has been ably followed by

Macleay and others. The resemblance, in the shape of the body and in the

fin-like anterior limbs, between the dugong, which is a pachydermatous

animal, and the whale, and between both these mammals and fishes, is

analogical. Amongst insects there are innumerable instances: thus

Linnaeus, misled by external appearances, actually classed an homopterous

insect as a moth. We see something of the same kind even in our domestic

varieties, as in the thickened stems of the common and swedish turnip. The

resemblance of the greyhound and racehorse is hardly more fanciful than the

analogies which have been drawn by some authors between very distinct

animals. On my view of characters being of real importance for

classification, only in so far as they reveal descent, we can clearly

understand why analogical or adaptive character, although of the utmost

importance to the welfare of the being, are almost valueless to the

systematist. For animals, belonging to two most distinct lines of descent,

may readily become adapted to similar conditions, and thus assume a close

external resemblance; but such resemblances will not reveal--will rather

tend to conceal their blood-relationship to their proper lines of descent.

We can also understand the apparent paradox, that the very same characters

are analogical when one class or order is compared with another, but give

true affinities when the members of the same class or order are compared

one with another: thus the shape of the body and fin-like limbs are only

analogical when whales are compared with fishes, being adaptations in both

classes for swimming through the water; but the shape of the body and

fin-like limbs serve as characters exhibiting true affinity between the

several members of the whale family; for these cetaceans agree in so many

characters, great and small, that we cannot doubt that they have inherited

their general shape of body and structure of limbs from a common ancestor.

So it is with fishes.

As members of distinct classes have often been adapted by successive slight

modifications to live under nearly similar circumstances,--to inhabit for

instance the three elements of land, air, and water,--we can perhaps

understand how it is that a numerical parallelism has sometimes been

observed between the sub-groups in distinct classes. A naturalist, struck

by a parallelism of this nature in any one class, by arbitrarily raising or

sinking the value of the groups in other classes (and all our experience

shows that this valuation has hitherto been arbitrary), could easily extend

the parallelism over a wide range; and thus the septenary, quinary,

quaternary, and ternary classifications have probably arisen.

As the modified descendants of dominant species, belonging to the larger

genera, tend to inherit the advantages, which made the groups to which they

belong large and their parents dominant, they are almost sure to spread

widely, and to seize on more and more places in the economy of nature. The

larger and more dominant groups thus tend to go on increasing in size; and

they consequently supplant many smaller and feebler groups. Thus we can

account for the fact that all organisms, recent and extinct, are included

under a few great orders, under still fewer classes, and all in one great

natural system. As showing how few the higher groups are in number, and

how widely spread they are throughout the world, the fact is striking, that

the discovery of Australia has not added a single insect belonging to a new

order; and that in the vegetable kingdom, as I learn from Dr. Hooker, it

has added only two or three orders of small size.

In the chapter on geological succession I attempted to show, on the

principle of each group having generally diverged much in character during

the long-continued process of modification, how it is that the more ancient

forms of life often present characters in some slight degree intermediate

between existing groups. A few old and intermediate parent-forms having

occasionally transmitted to the present day descendants but little

modified, will give to us our so-called osculant or aberrant groups. The

more aberrant any form is, the greater must be the number of connecting

forms which on my theory have been exterminated and utterly lost. And we

have some evidence of aberrant forms having suffered severely from

extinction, for they are generally represented by extremely few species;

and such species as do occur are generally very distinct from each other,

which again implies extinction. The genera Ornithorhynchus and

Lepidosiren, for example, would not have been less aberrant had each been

represented by a dozen species instead of by a single one; but such

richness in species, as I find after some investigation, does not commonly

fall to the lot of aberrant genera. We can, I think, account for this fact

only by looking at aberrant forms as failing groups conquered by more

successful competitors, with a few members preserved by some unusual

coincidence of favourable circumstances.

Mr. Waterhouse has remarked that, when a member belonging to one group of

animals exhibits an affinity to a quite distinct group, this affinity in

most cases is general and not special: thus, according to Mr. Waterhouse,

of all Rodents, the bizcacha is most nearly related to Marsupials; but in

the points in which it approaches this order, its relations are general,

and not to any one marsupial species more than to another. As the points

of affinity of the bizcacha to Marsupials are believed to be real and not

merely adaptive, they are due on my theory to inheritance in common.

Therefore we must suppose either that all Rodents, including the bizcacha,

branched off from some very ancient Marsupial, which will have had a

character in some degree intermediate with respect to all existing

Marsupials; or that both Rodents and Marsupials branched off from a common

progenitor, and that both groups have since undergone much modification in

divergent directions. On either view we may suppose that the bizcacha has

retained, by inheritance, more of the character of its ancient progenitor

than have other Rodents; and therefore it will not be specially related to

any one existing Marsupial, but indirectly to all or nearly all Marsupials,

from having partially retained the character of their common progenitor, or

of an early member of the group. On the other hand, of all Marsupials, as

Mr. Waterhouse has remarked, the phascolomys resembles most nearly, not any

one species, but the general order of Rodents. In this case, however, it

may be strongly suspected that the resemblance is only analogical, owing to

the phascolomys having become adapted to habits like those of a Rodent.

The elder De Candolle has made nearly similar observations on the general

nature of the affinities of distinct orders of plants.

On the principle of the multiplication and gradual divergence in character

of the species descended from a common parent, together with their

retention by inheritance of some characters in common, we can understand

the excessively complex and radiating affinities by which all the members

of the same family or higher group are connected together. For the common

parent of a whole family of species, now broken up by extinction into

distinct groups and sub-groups, will have transmitted some of its

characters, modified in various ways and degrees, to all; and the several

species will consequently be related to each other by circuitous lines of

affinity of various lengths (as may be seen in the diagram so often

referred to), mounting up through many predecessors. As it is difficult to

show the blood-relationship between the numerous kindred of any ancient and

noble family, even by the aid of a genealogical tree, and almost impossible

to do this without this aid, we can understand the extraordinary difficulty

which naturalists have experienced in describing, without the aid of a

diagram, the various affinities which they perceive between the many living

and extinct members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an important

part in defining and widening the intervals between the several groups in

each class. We may thus account even for the distinctness of whole classes

from each other--for instance, of birds from all other vertebrate

animals--by the belief that many ancient forms of life have been utterly

lost, through which the early progenitors of birds were formerly connected

with the early progenitors of the other vertebrate classes. There has been

less entire extinction of the forms of life which once connected fishes

with batrachians. There has been still less in some other classes, as in

that of the Crustacea, for here the most wonderfully diverse forms are

still tied together by a long, but broken, chain of affinities. Extinction

has only separated groups: it has by no means made them; for if every form

which has ever lived on this earth were suddenly to reappear, though it

would be quite impossible to give definitions by which each group could be

distinguished from other groups, as all would blend together by steps as

fine as those between the finest existing varieties, nevertheless a natural

classification, or at least a natural arrangement, would be possible. We

shall see this by turning to the diagram: the letters, A to L, may

represent eleven Silurian genera, some of which have produced large groups

of modified descendants. Every intermediate link between these eleven

genera and their primordial parent, and every intermediate link in each

branch and sub-branch of their descendants, may be supposed to be still

alive; and the links to be as fine as those between the finest varieties.

In this case it would be quite impossible to give any definition by which

the several members of the several groups could be distinguished from their

more immediate parents; or these parents from their ancient and unknown

progenitor. Yet the natural arrangement in the diagram would still hold

good; and, on the principle of inheritance, all the forms descended from A,

or from I, would have something in common. In a tree we can specify this

or that branch, though at the actual fork the two unite and blend together.

We could not, as I have said, define the several groups; but we could pick

out types, or forms, representing most of the characters of each group,

whether large or small, and thus give a general idea of the value of the

differences between them. This is what we should be driven to, if we were

ever to succeed in collecting all the forms in any class which have lived

throughout all time and space. We shall certainly never succeed in making

so perfect a collection: nevertheless, in certain classes, we are tending

in this direction; and Milne Edwards has lately insisted, in an able paper,

on the high importance of looking to types, whether or not we can separate

and define the groups to which such types belong.

Finally, we have seen that natural selection, which results from the

struggle for existence, and which almost inevitably induces extinction and

divergence of character in the many descendants from one dominant

parent-species, explains that great and universal feature in the affinities

of all organic beings, namely, their subordination in group under group.

We use the element of descent in classing the individuals of both sexes and

of all ages, although having few characters in common, under one species;

we use descent in classing acknowledged varieties, however different they

may be from their parent; and I believe this element of descent is the

hidden bond of connexion which naturalists have sought under the term of

the Natural System. On this idea of the natural system being, in so far as

it has been perfected, genealogical in its arrangement, with the grades of

difference between the descendants from a common parent, expressed by the

terms genera, families, orders, &c., we can understand the rules which we

are compelled to follow in our classification. We can understand why we

value certain resemblances far more than others; why we are permitted to

use rudimentary and useless organs, or others of trifling physiological

importance; why, in comparing one group with a distinct group, we summarily

reject analogical or adaptive characters, and yet use these same characters

within the limits of the same group. We can clearly see how it is that all

living and extinct forms can be grouped together in one great system; and

how the several members of each class are connected together by the most

complex and radiating lines of affinities. We shall never, probably,

disentangle the inextricable web of affinities between the members of any

one class; but when we have a distinct object in view, and do not look to

some unknown plan of creation, we may hope to make sure but slow progress.

Morphology. -- We have seen that the members of the same class,

independently of their habits of life, resemble each other in the general

plan of their organisation. This resemblance is often expressed by the

term 'unity of type;' or by saying that the several parts and organs in the

different species of the class are homologous. The whole subject is

included under the general name of Morphology. This is the most

interesting department of natural history, and may be said to be its very

soul. What can be more curious than that the hand of a man, formed for

grasping, that of a mole for digging, the leg of the horse, the paddle of

the porpoise, and the wing of the bat, should all be constructed on the

same pattern, and should include the same bones, in the same relative

positions? Geoffroy St. Hilaire has insisted strongly on the high

importance of relative connexion in homologous organs: the parts may

change to almost any extent in form and size, and yet they always remain

connected together in the same order. We never find, for instance, the

bones of the arm and forearm, or of the thigh and leg, transposed. Hence

the same names can be given to the homologous bones in widely different

animals. We see the same great law in the construction of the mouths of

insects: what can be more different than the immensely long spiral

proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the

great jaws of a beetle?--yet all these organs, serving for such different

purposes, are formed by infinitely numerous modifications of an upper lip,

mandibles, and two pairs of maxillae. Analogous laws govern the

construction of the mouths and limbs of crustaceans. So it is with the

flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity of

pattern in members of the same class, by utility or by the doctrine of

final causes. The hopelessness of the attempt has been expressly admitted

by Owen in his most interesting work on the 'Nature of Limbs.' On the

ordinary view of the independent creation of each being, we can only say

that so it is;--that it has so pleased the Creator to construct each animal

and plant.

The explanation is manifest on the theory of the natural selection of

successive slight modifications,--each modification being profitable in

some way to the modified form, but often affecting by correlation of growth

other parts of the organisation. In changes of this nature, there will be

little or no tendency to modify the original pattern, or to transpose

parts. The bones of a limb might be shortened and widened to any extent,

and become gradually enveloped in thick membrane, so as to serve as a fin;

or a webbed foot might have all its bones, or certain bones, lengthened to

any extent, and the membrane connecting them increased to any extent, so as

to serve as a wing: yet in all this great amount of modification there

will be no tendency to alter the framework of bones or the relative

connexion of the several parts. If we suppose that the ancient progenitor,

the archetype as it may be called, of all mammals, had its limbs

constructed on the existing general pattern, for whatever purpose they

served, we can at once perceive the plain signification of the homologous

construction of the limbs throughout the whole class. So with the mouths

of insects, we have only to suppose that their common progenitor had an

upper lip, mandibles, and two pair of maxillae, these parts being perhaps

very simple in form; and then natural selection will account for the

infinite diversity in structure and function of the mouths of insects.

Nevertheless, it is conceivable that the general pattern of an organ might

become so much obscured as to be finally lost, by the atrophy and

ultimately by the complete abortion of certain parts, by the soldering

together of other parts, and by the doubling or multiplication of

others,--variations which we know to be within the limits of possibility.

In the paddles of the extinct gigantic sea-lizards, and in the mouths of

certain suctorial crustaceans, the general pattern seems to have been thus

to a certain extent obscured.

There is another and equally curious branch of the present subject; namely,

the comparison not of the same part in different members of a class, but of

the different parts or organs in the same individual. Most physiologists

believe that the bones of the skull are homologous with--that is correspond

in number and in relative connexion with--the elemental parts of a certain

number of vertebrae. The anterior and posterior limbs in each member of

the vertebrate and articulate classes are plainly homologous. We see the

same law in comparing the wonderfully complex jaws and legs in crustaceans.

It is familiar to almost every one, that in a flower the relative position

of the sepals, petals, stamens, and pistils, as well as their intimate

structure, are intelligible on the view that they consist of metamorphosed

leaves, arranged in a spire. In monstrous plants, we often get direct

evidence of the possibility of one organ being transformed into another;

and we can actually see in embryonic crustaceans and in many other animals,

and in flowers, that organs, which when mature become extremely different,

are at an early stage of growth exactly alike.

How inexplicable are these facts on the ordinary view of creation! Why

should the brain be enclosed in a box composed of such numerous and such

extraordinarily shaped pieces of bone? As Owen has remarked, the benefit

derived from the yielding of the separate pieces in the act of parturition

of mammals, will by no means explain the same construction in the skulls of

birds. Why should similar bones have been created in the formation of the

wing and leg of a bat, used as they are for such totally different

purposes? Why should one crustacean, which has an extremely complex mouth

formed of many parts, consequently always have fewer legs; or conversely,

those with many legs have simpler mouths? Why should the sepals, petals,

stamens, and pistils in any individual flower, though fitted for such

widely different purposes, be all constructed on the same pattern?

On the theory of natural selection, we can satisfactorily answer these

questions. In the vertebrata, we see a series of internal vertebrae

bearing certain processes and appendages; in the articulata, we see the

body divided into a series of segments, bearing external appendages; and in

flowering plants, we see a series of successive spiral whorls of leaves.

An indefinite repetition of the same part or organ is the common

characteristic (as Owen has observed) of all low or little-modified forms;

therefore we may readily believe that the unknown progenitor of the

vertebrata possessed many vertebrae; the unknown progenitor of the

articulata, many segments; and the unknown progenitor of flowering plants,

many spiral whorls of leaves. We have formerly seen that parts many times

repeated are eminently liable to vary in number and structure; consequently

it is quite probable that natural selection, during a long-continued course

of modification, should have seized on a certain number of the primordially

similar elements, many times repeated, and have adapted them to the most

diverse purposes. And as the whole amount of modification will have been

effected by slight successive steps, we need not wonder at discovering in

such parts or organs, a certain degree of fundamental resemblance, retained

by the strong principle of inheritance.

In the great class of molluscs, though we can homologise the parts of one

species with those of another and distinct species, we can indicate but few

serial homologies; that is, we are seldom enabled to say that one part or

organ is homologous with another in the same individual. And we can

understand this fact; for in molluscs, even in the lowest members of the

class, we do not find nearly so much indefinite repetition of any one part,

as we find in the other great classes of the animal and vegetable kingdoms.

Naturalists frequently speak of the skull as formed of metamorphosed

vertebrae: the jaws of crabs as metamorphosed legs; the stamens and

pistils of flowers as metamorphosed leaves; but it would in these cases

probably be more correct, as Professor Huxley has remarked, to speak of

both skull and vertebrae, both jaws and legs, &c.,--as having been

metamorphosed, not one from the other, but from some common element.

Naturalists, however, use such language only in a metaphorical sense: they

are far from meaning that during a long course of descent, primordial

organs of any kind--vertebrae in the one case and legs in the other--have

actually been modified into skulls or jaws. Yet so strong is the

appearance of a modification of this nature having occurred, that

naturalists can hardly avoid employing language having this plain

signification. On my view these terms may be used literally; and the

wonderful fact of the jaws, for instance, of a crab retaining numerous

characters, which they would probably have retained through inheritance, if

they had really been metamorphosed during a long course of descent from

true legs, or from some simple appendage, is explained.

Embryology. -- It has already been casually remarked that certain organs in

the individual, which when mature become widely different and serve for

different purposes, are in the embryo exactly alike. The embryos, also, of

distinct animals within the same class are often strikingly similar: a

better proof of this cannot be given, than a circumstance mentioned by

Agassiz, namely, that having forgotten to ticket the embryo of some

vertebrate animal, he cannot now tell whether it be that of a mammal, bird,

or reptile. The vermiform larvae of moths, flies, beetles, &c., resemble

each other much more closely than do the mature insects; but in the case of

larvae, the embryos are active, and have been adapted for special lines of

life. A trace of the law of embryonic resemblance, sometimes lasts till a

rather late age: thus birds of the same genus, and of closely allied

genera, often resemble each other in their first and second plumage; as we

see in the spotted feathers in the thrush group. In the cat tribe, most of

the species are striped or spotted in lines; and stripes can be plainly

distinguished in the whelp of the lion. We occasionally though rarely see

something of this kind in plants: thus the embryonic leaves of the ulex or

furze, and the first leaves of the phyllodineous acaceas, are pinnate or

divided like the ordinary leaves of the leguminosae.

The points of structure, in which the embryos of widely different animals

of the same class resemble each other, often have no direct relation to

their conditions of existence. We cannot, for instance, suppose that in

the embryos of the vertebrata the peculiar loop-like course of the arteries

near the branchial slits are related to similar conditions,--in the young

mammal which is nourished in the womb of its mother, in the egg of the bird

which is hatched in a nest, and in the spawn of a frog under water. We

have no more reason to believe in such a relation, than we have to believe

that the same bones in the hand of a man, wing of a bat, and fin of a

porpoise, are related to similar conditions of life. No one will suppose

that the stripes on the whelp of a lion, or the spots on the young

blackbird, are of any use to these animals, or are related to the

conditions to which they are exposed.

The case, however, is different when an animal during any part of its

embryonic career is active, and has to provide for itself. The period of

activity may come on earlier or later in life; but whenever it comes on,

the adaptation of the larva to its conditions of life is just as perfect

and as beautiful as in the adult animal. From such special adaptations,

the similarity of the larvae or active embryos of allied animals is

sometimes much obscured; and cases could be given of the larvae of two

species, or of two groups of species, differing quite as much, or even

more, from each other than do their adult parents. In most cases, however,

the larvae, though active, still obey more or less closely the law of

common embryonic resemblance. Cirripedes afford a good instance of this:

even the illustrious Cuvier did not perceive that a barnacle was, as it

certainly is, a crustacean; but a glance at the larva shows this to be the

case in an unmistakeable manner. So again the two main divisions of

cirripedes, the pedunculated and sessile, which differ widely in external

appearance, have larvae in all their several stages barely distinguishable.

The embryo in the course of development generally rises in organisation: I

use this expression, though I am aware that it is hardly possible to define

clearly what is meant by the organisation being higher or lower. But no

one probably will dispute that the butterfly is higher than the

caterpillar. In some cases, however, the mature animal is generally

considered as lower in the scale than the larva, as with certain parasitic

crustaceans. To refer once again to cirripedes: the larvae in the first

stage have three pairs of legs, a very simple single eye, and a

probosciformed mouth, with which they feed largely, for they increase much

in size. In the second stage, answering to the chrysalis stage of

butterflies, they have six pairs of beautifully constructed natatory legs,

a pair of magnificent compound eyes, and extremely complex antennae; but

they have a closed and imperfect mouth, and cannot feed: their function at

this stage is, to search by their well-developed organs of sense, and to

reach by their active powers of swimming, a proper place on which to become

attached and to undergo their final metamorphosis. When this is completed

they are fixed for life: their legs are now converted into prehensile

organs; they again obtain a well-constructed mouth; but they have no

antennae, and their two eyes are now reconverted into a minute, single, and

very simple eye-spot. In this last and complete state, cirripedes may be

considered as either more highly or more lowly organised than they were in

the larval condition. But in some genera the larvae become developed

either into hermaphrodites having the ordinary structure, or into what I

have called complemental males: and in the latter, the development has

assuredly been retrograde; for the male is a mere sack, which lives for a

short time, and is destitute of mouth, stomach, or other organ of

importance, excepting for reproduction.

We are so much accustomed to see differences in structure between the

embryo and the adult, and likewise a close similarity in the embryos of

widely different animals within the same class, that we might be led to

look at these facts as necessarily contingent in some manner on growth.

But there is no obvious reason why, for instance, the wing of a bat, or the

fin of a porpoise, should not have been sketched out with all the parts in

proper proportion, as soon as any structure became visible in the embryo.

And in some whole groups of animals and in certain members of other groups,

the embryo does not at any period differ widely from the adult: thus Owen

has remarked in regard to cuttle-fish, 'there is no metamorphosis; the

cephalopodic character is manifested long before the parts of the embryo

are completed;' and again in spiders, 'there is nothing worthy to be called

a metamorphosis.' The larvae of insects, whether adapted to the most

diverse and active habits, or quite inactive, being fed by their parents or

placed in the midst of proper nutriment, yet nearly all pass through a

similar worm-like stage of development; but in some few cases, as in that

of Aphis, if we look to the admirable drawings by Professor Huxley of the

development of this insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology,--namely the

very general, but not universal difference in structure between the embryo

and the adult;--of parts in the same individual embryo, which ultimately

become very unlike and serve for diverse purposes, being at this early

period of growth alike;--of embryos of different species within the same

class, generally, but not universally, resembling each other;--of the

structure of the embryo not being closely related to its conditions of

existence, except when the embryo becomes at any period of life active and

has to provide for itself;--of the embryo apparently having sometimes a

higher organisation than the mature animal, into which it is developed. I

believe that all these facts can be explained, as follows, on the view of

descent with modification.

It is commonly assumed, perhaps from monstrosities often affecting the

embryo at a very early period, that slight variations necessarily appear at

an equally early period. But we have little evidence on this head--indeed

the evidence rather points the other way; for it is notorious that breeders

of cattle, horses, and various fancy animals, cannot positively tell, until

some time after the animal has been born, what its merits or form will

ultimately turn out. We see this plainly in our own children; we cannot

always tell whether the child will be tall or short, or what its precise

features will be. The question is not, at what period of life any

variation has been caused, but at what period it is fully displayed. The

cause may have acted, and I believe generally has acted, even before the

embryo is formed; and the variation may be due to the male and female

sexual elements having been affected by the conditions to which either

parent, or their ancestors, have been exposed. Nevertheless an effect thus

caused at a very early period, even before the formation of the embryo, may

appear late in life; as when an hereditary disease, which appears in old

age alone, has been communicated to the offspring from the reproductive

element of one parent. Or again, as when the horns of cross-bred cattle

have been affected by the shape of the horns of either parent. For the

welfare of a very young animal, as long as it remains in its mother's womb,

or in the egg, or as long as it is nourished and protected by its parent,

it must be quite unimportant whether most of its characters are fully

acquired a little earlier or later in life. It would not signify, for

instance, to a bird which obtained its food best by having a long beak,

whether or not it assumed a beak of this particular length, as long as it

was fed by its parents. Hence, I conclude, that it is quite possible, that

each of the many successive modifications, by which each species has

acquired its present structure, may have supervened at a not very early

period of life; and some direct evidence from our domestic animals supports

this view. But in other cases it is quite possible that each successive

modification, or most of them, may have appeared at an extremely early

period.

I have stated in the first chapter, that there is some evidence to render

it probable, that at whatever age any variation first appears in the

parent, it tends to reappear at a corresponding age in the offspring.

Certain variations can only appear at corresponding ages, for instance,

peculiarities in the caterpillar, cocoon, or imago states of the silk-moth;

or, again, in the horns of almost full-grown cattle. But further than

this, variations which, for all that we can see, might have appeared

earlier or later in life, tend to appear at a corresponding age in the

offspring and parent. I am far from meaning that this is invariably the

case; and I could give a good many cases of variations (taking the word in

the largest sense) which have supervened at an earlier age in the child

than in the parent.

These two principles, if their truth be admitted, will, I believe, explain

all the above specified leading facts in embryology. But first let us look

at a few analogous cases in domestic varieties. Some authors who have

written on Dogs, maintain that the greyhound and bulldog, though appearing

so different, are really varieties most closely allied, and have probably

descended from the same wild stock; hence I was curious to see how far

their puppies differed from each other: I was told by breeders that they

differed just as much as their parents, and this, judging by the eye,

seemed almost to be the case; but on actually measuring the old dogs and

their six-days old puppies, I found that the puppies had not nearly

acquired their full amount of proportional difference. So, again, I was

told that the foals of cart and race-horses differed as much as the

full-grown animals; and this surprised me greatly, as I think it probable

that the difference between these two breeds has been wholly caused by

selection under domestication; but having had careful measurements made of

the dam and of a three-days old colt of a race and heavy cart-horse, I find

that the colts have by no means acquired their full amount of proportional

difference.

As the evidence appears to me conclusive, that the several domestic breeds

of Pigeon have descended from one wild species, I compared young pigeons of

various breeds, within twelve hours after being hatched; I carefully

measured the proportions (but will not here give details) of the beak,

width of mouth, length of nostril and of eyelid, size of feet and length of

leg, in the wild stock, in pouters, fantails, runts, barbs, dragons,

carriers, and tumblers. Now some of these birds, when mature, differ so

extraordinarily in length and form of beak, that they would, I cannot

doubt, be ranked in distinct genera, had they been natural productions.

But when the nestling birds of these several breeds were placed in a row,

though most of them could be distinguished from each other, yet their

proportional differences in the above specified several points were

incomparably less than in the full-grown birds. Some characteristic points

of difference--for instance, that of the width of mouth--could hardly be

detected in the young. But there was one remarkable exception to this

rule, for the young of the short-faced tumbler differed from the young of

the wild rock-pigeon and of the other breeds, in all its proportions,

almost exactly as much as in the adult state.

The two principles above given seem to me to explain these facts in regard

to the later embryonic stages of our domestic varieties. Fanciers select

their horses, dogs, and pigeons, for breeding, when they are nearly grown

up: they are indifferent whether the desired qualities and structures have

been acquired earlier or later in life, if the full-grown animal possesses

them. And the cases just given, more especially that of pigeons, seem to

show that the characteristic differences which give value to each breed,

and which have been accumulated by man's selection, have not generally

first appeared at an early period of life, and have been inherited by the

offspring at a corresponding not early period. But the case of the

short-faced tumbler, which when twelve hours old had acquired its proper

proportions, proves that this is not the universal rule; for here the

characteristic differences must either have appeared at an earlier period

than usual, or, if not so, the differences must have been inherited, not at

the corresponding, but at an earlier age.

Now let us apply these facts and the above two principles--which latter,

though not proved true, can be shown to be in some degree probable--to

species in a state of nature. Let us take a genus of birds, descended on

my theory from some one parent-species, and of which the several new

species have become modified through natural selection in accordance with

their diverse habits. Then, from the many slight successive steps of

variation having supervened at a rather late age, and having been inherited

at a corresponding age, the young of the new species of our supposed genus

will manifestly tend to resemble each other much more closely than do the

adults, just as we have seen in the case of pigeons. We may extend this

view to whole families or even classes. The fore-limbs, for instance,

which served as legs in the parent-species, may become, by a long course of

modification, adapted in one descendant to act as hands, in another as

paddles, in another as wings; and on the above two principles--namely of

each successive modification supervening at a rather late age, and being

inherited at a corresponding late age--the fore-limbs in the embryos of the

several descendants of the parent-species will still resemble each other

closely, for they will not have been modified. But in each individual new

species, the embryonic fore-limbs will differ greatly from the fore-limbs

in the mature animal; the limbs in the latter having undergone much

modification at a rather late period of life, and having thus been

converted into hands, or paddles, or wings. Whatever influence

long-continued exercise or use on the one hand, and disuse on the other,

may have in modifying an organ, such influence will mainly affect the

mature animal, which has come to its full powers of activity and has to

gain its own living; and the effects thus produced will be inherited at a

corresponding mature age. Whereas the young will remain unmodified, or be

modified in a lesser degree, by the effects of use and disuse.

In certain cases the successive steps of variation might supervene, from

causes of which we are wholly ignorant, at a very early period of life, or

each step might be inherited at an earlier period than that at which it

first appeared. In either case (as with the short-faced tumbler) the young

or embryo would closely resemble the mature parent-form. We have seen that

this is the rule of development in certain whole groups of animals, as with

cuttle-fish and spiders, and with a few members of the great class of

insects, as with Aphis. With respect to the final cause of the young in

these cases not undergoing any metamorphosis, or closely resembling their

parents from their earliest age, we can see that this would result from the

two following contingencies; firstly, from the young, during a course of

modification carried on for many generations, having to provide for their

own wants at a very early stage of development, and secondly, from their

following exactly the same habits of life with their parents; for in this

case, it would be indispensable for the existence of the species, that the

child should be modified at a very early age in the same manner with its

parents, in accordance with their similar habits. Some further

explanation, however, of the embryo not undergoing any metamorphosis is

perhaps requisite. If, on the other hand, it profited the young to follow

habits of life in any degree different from those of their parent, and

consequently to be constructed in a slightly different manner, then, on the

principle of inheritance at corresponding ages, the active young or larvae

might easily be rendered by natural selection different to any conceivable

extent from their parents. Such differences might, also, become correlated

with successive stages of development; so that the larvae, in the first

stage, might differ greatly from the larvae in the second stage, as we have

seen to be the case with cirripedes. The adult might become fitted for

sites or habits, in which organs of locomotion or of the senses, &c., would

be useless; and in this case the final metamorphosis would be said to be

retrograde.

As all the organic beings, extinct and recent, which have ever lived on

this earth have to be classed together, and as all have been connected by

the finest gradations, the best, or indeed, if our collections were nearly

perfect, the only possible arrangement, would be genealogical. Descent

being on my view the hidden bond of connexion which naturalists have been

seeking under the term of the natural system. On this view we can

understand how it is that, in the eyes of most naturalists, the structure

of the embryo is even more important for classification than that of the

adult. For the embryo is the animal in its less modified state; and in so

far it reveals the structure of its progenitor. In two groups of animal,

however much they may at present differ from each other in structure and

habits, if they pass through the same or similar embryonic stages, we may

feel assured that they have both descended from the same or nearly similar

parents, and are therefore in that degree closely related. Thus, community

in embryonic structure reveals community of descent. It will reveal this

community of descent, however much the structure of the adult may have been

modified and obscured; we have seen, for instance, that cirripedes can at

once be recognised by their larvae as belonging to the great class of

crustaceans. As the embryonic state of each species and group of species

partially shows us the structure of their less modified ancient

progenitors, we can clearly see why ancient and extinct forms of life

should resemble the embryos of their descendants,--our existing species.

Agassiz believes this to be a law of nature; but I am bound to confess that

I only hope to see the law hereafter proved true. It can be proved true in

those cases alone in which the ancient state, now supposed to be

represented in many embryos, has not been obliterated, either by the

successive variations in a long course of modification having supervened at

a very early age, or by the variations having been inherited at an earlier

period than that at which they first appeared. It should also be borne in

mind, that the supposed law of resemblance of ancient forms of life to the

embryonic stages of recent forms, may be true, but yet, owing to the

geological record not extending far enough back in time, may remain for a

long period, or for ever, incapable of demonstration.

Thus, as it seems to me, the leading facts in embryology, which are second

in importance to none in natural history, are explained on the principle of

slight modifications not appearing, in the many descendants from some one

ancient progenitor, at a very early period in the life of each, though

perhaps caused at the earliest, and being inherited at a corresponding not

early period. Embryology rises greatly in interest, when we thus look at

the embryo as a picture, more or less obscured, of the common parent-form

of each great class of animals.

Rudimentary, atrophied, or aborted organs. -- Organs or parts in this

strange condition, bearing the stamp of inutility, are extremely common

throughout nature. For instance, rudimentary mammae are very general in

the males of mammals: I presume that the 'bastard-wing' in birds may be

safely considered as a digit in a rudimentary state: in very many snakes

one lobe of the lungs is rudimentary; in other snakes there are rudiments

of the pelvis and hind limbs. Some of the cases of rudimentary organs are

extremely curious; for instance, the presence of teeth in foetal whales,

which when grown up have not a tooth in their heads; and the presence of

teeth, which never cut through the gums, in the upper jaws of our unborn

calves. It has even been stated on good authority that rudiments of teeth

can be detected in the beaks of certain embryonic birds. Nothing can be

plainer than that wings are formed for flight, yet in how many insects do

we see wings so reduced in size as to be utterly incapable of flight, and

not rarely lying under wing-cases, firmly soldered together!

The meaning of rudimentary organs is often quite unmistakeable: for

instance there are beetles of the same genus (and even of the same species)

resembling each other most closely in all respects, one of which will have

full-sized wings, and another mere rudiments of membrane; and here it is

impossible to doubt, that the rudiments represent wings. Rudimentary

organs sometimes retain their potentiality, and are merely not developed:

this seems to be the case with the mammae of male mammals, for many

instances are on record of these organs having become well developed in

full-grown males, and having secreted milk. So again there are normally

four developed and two rudimentary teats in the udders of the genus Bos,

but in our domestic cows the two sometimes become developed and give milk.

In individual plants of the same species the petals sometimes occur as mere

rudiments, and sometimes in a well-developed state. In plants with

separated sexes, the male flowers often have a rudiment of a pistil; and

Kolreuter found that by crossing such male plants with an hermaphrodite

species, the rudiment of the pistil in the hybrid offspring was much

increased in size; and this shows that the rudiment and the perfect pistil

are essentially alike in nature.

An organ serving for two purposes, may become rudimentary or utterly

aborted for one, even the more important purpose; and remain perfectly

efficient for the other. Thus in plants, the office of the pistil is to

allow the pollen-tubes to reach the ovules protected in the ovarium at its

base. The pistil consists of a stigma supported on the style; but in some

Compositae, the male florets, which of course cannot be fecundated, have a

pistil, which is in a rudimentary state, for it is not crowned with a

stigma; but the style remains well developed, and is clothed with hairs as

in other compositae, for the purpose of brushing the pollen out of the

surrounding anthers. Again, an organ may become rudimentary for its proper

purpose, and be used for a distinct object: in certain fish the

swim-bladder seems to be rudimentary for its proper function of giving

buoyancy, but has become converted into a nascent breathing organ or lung.

Other similar instances could be given.

Rudimentary organs in the individuals of the same species are very liable

to vary in degree of development and in other respects. Moreover, in

closely allied species, the degree to which the same organ has been

rendered rudimentary occasionally differs much. This latter fact is well

exemplified in the state of the wings of the female moths in certain

groups. Rudimentary organs may be utterly aborted; and this implies, that

we find in an animal or plant no trace of an organ, which analogy would

lead us to expect to find, and which is occasionally found in monstrous

individuals of the species. Thus in the snapdragon (antirrhinum) we

generally do not find a rudiment of a fifth stamen; but this may sometimes

be seen. In tracing the homologies of the same part in different members

of a class, nothing is more common, or more necessary, than the use and

discovery of rudiments. This is well shown in the drawings given by Owen

of the bones of the leg of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the upper

jaws of whales and ruminants, can often be detected in the embryo, but

afterwards wholly disappear. It is also, I believe, a universal rule, that

a rudimentary part or organ is of greater size relatively to the adjoining

parts in the embryo, than in the adult; so that the organ at this early age

is less rudimentary, or even cannot be said to be in any degree

rudimentary. Hence, also, a rudimentary organ in the adult, is often said

to have retained its embryonic condition.

I have now given the leading facts with respect to rudimentary organs. In

reflecting on them, every one must be struck with astonishment: for the

same reasoning power which tells us plainly that most parts and organs are

exquisitely adapted for certain purposes, tells us with equal plainness

that these rudimentary or atrophied organs, are imperfect and useless. In

works on natural history rudimentary organs are generally said to have been

created 'for the sake of symmetry,' or in order 'to complete the scheme of

nature;' but this seems to me no explanation, merely a restatement of the

fact. Would it be thought sufficient to say that because planets revolve

in elliptic courses round the sun, satellites follow the same course round

the planets, for the sake of symmetry, and to complete the scheme of

nature? An eminent physiologist accounts for the presence of rudimentary

organs, by supposing that they serve to excrete matter in excess, or

injurious to the system; but can we suppose that the minute papilla, which

often represents the pistil in male flowers, and which is formed merely of

cellular tissue, can thus act? Can we suppose that the formation of

rudimentary teeth which are subsequently absorbed, can be of any service to

the rapidly growing embryonic calf by the excretion of precious phosphate

of lime? When a man's fingers have been amputated, imperfect nails

sometimes appear on the stumps: I could as soon believe that these

vestiges of nails have appeared, not from unknown laws of growth, but in

order to excrete horny matter, as that the rudimentary nails on the fin of

the manatee were formed for this purpose.

On my view of descent with modification, the origin of rudimentary organs

is simple. We have plenty of cases of rudimentary organs in our domestic

productions,--as the stump of a tail in tailless breeds,--the vestige of an

ear in earless breeds,--the reappearance of minute dangling horns in

hornless breeds of cattle, more especially, according to Youatt, in young

animals,--and the state of the whole flower in the cauliflower. We often

see rudiments of various parts in monsters. But I doubt whether any of

these cases throw light on the origin of rudimentary organs in a state of

nature, further than by showing that rudiments can be produced; for I doubt

whether species under nature ever undergo abrupt changes. I believe that

disuse has been the main agency; that it has led in successive generations

to the gradual reduction of various organs, until they have become

rudimentary,--as in the case of the eyes of animals inhabiting dark

caverns, and of the wings of birds inhabiting oceanic islands, which have

seldom been forced to take flight, and have ultimately lost the power of

flying. Again, an organ useful under certain conditions, might become

injurious under others, as with the wings of beetles living on small and

exposed islands; and in this case natural selection would continue slowly

to reduce the organ, until it was rendered harmless and rudimentary.

Any change in function, which can be effected by insensibly small steps, is

within the power of natural selection; so that an organ rendered, during

changed habits of life, useless or injurious for one purpose, might easily

be modified and used for another purpose. Or an organ might be retained

for one alone of its former functions. An organ, when rendered useless,

may well be variable, for its variations cannot be checked by natural

selection. At whatever period of life disuse or selection reduces an

organ, and this will generally be when the being has come to maturity and

to its full powers of action, the principle of inheritance at corresponding

ages will reproduce the organ in its reduced state at the same age, and

consequently will seldom affect or reduce it in the embryo. Thus we can

understand the greater relative size of rudimentary organs in the embryo,

and their lesser relative size in the adult. But if each step of the

process of reduction were to be inherited, not at the corresponding age,

but at an extremely early period of life (as we have good reason to believe

to be possible) the rudimentary part would tend to be wholly lost, and we

should have a case of complete abortion. The principle, also, of economy,

explained in a former chapter, by which the materials forming any part or

structure, if not useful to the possessor, will be saved as far as is

possible, will probably often come into play; and this will tend to cause

the entire obliteration of a rudimentary organ.

As the presence of rudimentary organs is thus due to the tendency in every

part of the organisation, which has long existed, to be inherited--we can

understand, on the genealogical view of classification, how it is that

systematists have found rudimentary parts as useful as, or even sometimes

more useful than, parts of high physiological importance. Rudimentary

organs may be compared with the letters in a word, still retained in the

spelling, but become useless in the pronunciation, but which serve as a

clue in seeking for its derivation. On the view of descent with

modification, we may conclude that the existence of organs in a

rudimentary, imperfect, and useless condition, or quite aborted, far from

presenting a strange difficulty, as they assuredly do on the ordinary

doctrine of creation, might even have been anticipated, and can be

accounted for by the laws of inheritance.

Summary. -- In this chapter I have attempted to show, that the

subordination of group to group in all organisms throughout all time; that

the nature of the relationship, by which all living and extinct beings are

united by complex, radiating, and circuitous lines of affinities into one

grand system; the rules followed and the difficulties encountered by

naturalists in their classifications; the value set upon characters, if

constant and prevalent, whether of high vital importance, or of the most

trifling importance, or, as in rudimentary organs, of no importance; the

wide opposition in value between analogical or adaptive characters, and

characters of true affinity; and other such rules;--all naturally follow on

the view of the common parentage of those forms which are considered by

naturalists as allied, together with their modification through natural

selection, with its contingencies of extinction and divergence of

character. In considering this view of classification, it should be borne

in mind that the element of descent has been universally used in ranking

together the sexes, ages, and acknowledged varieties of the same species,

however different they may be in structure. If we extend the use of this

element of descent,--the only certainly known cause of similarity in

organic beings,--we shall understand what is meant by the natural system:

it is genealogical in its attempted arrangement, with the grades of

acquired difference marked by the terms varieties, species, genera,

families, orders, and classes.

On this same view of descent with modification, all the great facts in

Morphology become intelligible,--whether we look to the same pattern

displayed in the homologous organs, to whatever purpose applied, of the

different species of a class; or to the homologous parts constructed on the

same pattern in each individual animal and plant.

On the principle of successive slight variations, not necessarily or

generally supervening at a very early period of life, and being inherited

at a corresponding period, we can understand the great leading facts in

Embryology; namely, the resemblance in an individual embryo of the

homologous parts, which when matured will become widely different from each

other in structure and function; and the resemblance in different species

of a class of the homologous parts or organs, though fitted in the adult

members for purposes as different as possible. Larvae are active embryos,

which have become specially modified in relation to their habits of life,

through the principle of modifications being inherited at corresponding

ages. On this same principle--and bearing in mind, that when organs are

reduced in size, either from disuse or selection, it will generally be at

that period of life when the being has to provide for its own wants, and

bearing in mind how strong is the principle of inheritance--the occurrence

of rudimentary organs and their final abortion, present to us no

inexplicable difficulties; on the contrary, their presence might have been

even anticipated. The importance of embryological characters and of

rudimentary organs in classification is intelligible, on the view that an

arrangement is only so far natural as it is genealogical.

Finally, the several classes of facts which have been considered in this

chapter, seem to me to proclaim so plainly, that the innumerable species,

genera, and families of organic beings, with which this world is peopled,

have all descended, each within its own class or group, from common

parents, and have all been modified in the course of descent, that I should

without hesitation adopt this view, even if it were unsupported by other

facts or arguments.