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 Linnæus, 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 Proteaceæ, 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

Connaraceæ “differ in having one or more ovaria, in the existence or

absence of albumen, in the imbricate or valvular æstivation. 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 antennæ, 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 antennæ 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 Algæ—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 Linnæus, 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 Malpighiaceæ, 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 Malpighiaceæ. 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 ærating 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 (_a_14 to _z_14) 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, etc.; 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 larvæ

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 Linnæus, 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, etc., 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 maxillæ. 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 maxillæ, 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 vertebræ. 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 vertebræ

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 vertebræ; 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

vertebræ: 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 vertebræ, both jaws and legs, etc.,—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—vertebræ 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 larvæ of moths, flies,

beetles, etc., resemble each other much more closely than do the mature

insects; but in the case of larvæ, 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 leguminosæ.

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 larvæ or active embryos of allied

animals is sometimes much obscured; and cases could be given of the

larvæ 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 larvæ, 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 larvæ 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 larvæ 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 antennæ; 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

antennæ, 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 larvæ 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 larvæ 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 larvæ 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 larvæ, in the first

stage, might differ greatly from the larvæ 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,

etc., 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 larvæ 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 mammæ 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 mammæ 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 Kölreuter 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 Compositæ, 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

compositæ, 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. Larvæ 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.