The Norwegians, Swedes, Danes, Germans, Dutch, and Anglo-Saxons, form together a group of Scandinavian races, that are but slightly divergent in their characters. Welsh, Irish, and Highlanders, though they have difierences, have not differences such as to hide a decided community of na- ture: they are classed together as Celts. Between the Scandinavian race as a whole and the Celtic race as a whole, there is a recognized distinction greater than that between the sub-divisions which make up one or the other. And the several peoples inhabiting Southern Europe are more nearly allied to one another, than the aggregate they form is allied to the aggregates of JN^orthern peoples. If, again, we compare these European varieties of man taken as a group with that group of Eastern varieties which had a common origin with it, we see a stronger contrast than between the European varieties themselves. And once more, ethnolo- gists find difierences of still higher importance, between the Aryan stock as a whole and the Mongolian stock as a whole, THE ARGUMENTS FROM (CLASSIFICATION. 357 or the NcgTo stock as a whole. Tliough these contrasts are partially obscured by intermixtures; yet they are not so obscured as to hide the truths that the most-nearly-allied varieties of man, are those which diverged from one ano- ther at a comparatively-recent period; that each group of nearly- allied varieties, is more strongly contrasted with other such groups that had a common origin with it at a remoter period; and so on, until we come to the largest groups, which are the most strongly contrasted, and of whose divergence no trace is extant.
The relations existing among the classes and sub-classes of languages, have been briefly referred to by Mr Darwin, in illustration of his argmnent. We know that languages have arisen by evolution. Let us then see what grouping of them evolution has produced. On comparing the dialects of adja- cent counties in England, we find that their diiferences are so small as scarcely to distinguish them. Between the dialects of the Northern counties taken together, and those of the Southern counties taken together, the contrast is stronger. These clusters of dialects, together with those of Scotland and Ireland, are nevertheless so similar, that we regard them as one language. The several languages of Scandinavian Eu- rope, including English, are much more unlike one ano- ther, than are the several dialects which each of them in- cludes; in correspondence with the fact that they diverged from one another at earlier periods than did their respective dialects. The Scandinavian languages have nevertheless a certain community of character, which distinguishes them as a group from the languages of Southern Europe; between which there are general and special affinities that similarly unite them into a group formed of sub-groups containing sub- sub-groups. And this wider divergence between the order of languages spoken in Northern Europe, and the order of languages spoken in Southern Europe, answers to the longer time that has elapsed since their difierentiation commenced. Further, these two orders of modern European languages, as 358 THE EVOLUTION OF LIFE.
well as Latin and Greek and certain extinct and spoken languages of the East, are shown to have traits in common, which, notwithstanding the wide gaps between them, unite them together as one great class of Aryan lan- guages; radically distinguished from the classes of lan- guages spoken by the other great divisions of the human race.
§ 123. Now this kind of subordination of groups, which we see arises in the course of continuous descent, multiplica- tion, and divergence, is just the kind of subordination of groups which plants and animals exhibit: it is just this kind of subordination which has thrust itself on the attention of naturalists, in spite of pre-conceptions.
The original idea was that of arrangement in linear order. We saw that even after a considerable acquaintance with the structures of organisms had been acquired, naturalists con- tinued their efforts to reconcile the facts with the notion of a uni-serial succession. The accumulation of evidence necessi- tated the breaking up of the imagined chain into groups and sub-groups. Gradually there arose the conviction that these groups do not admit of being placed in a line. And the conception finally arrived at, is, that of certain great sub- kingdoms, very widely divergent, each made up of classes much less widely divergent, severally containing orders still less divergent; and so on with genera and species. The diagram on page 303, shows the general relations of these divisions in their degrees of subordination.
Hence this " grand fact in natural history of the subordina- tion of group under group, which from its familiarity does not always sufficiently strike us," is perfectly in harmony with the hypothesis of evolution. The extreme significance of this kind of relation among organic forms, is dwelt on by Mr Darwin; who shows how an ordinary genealogical tree represents, on a small scale, a system of grouping analogous to that which exists among organisms in general, and which is _j _j THE ARGUMENTS FROM CLASSIFICATION. 359 explained on the supposition of a genealogical tree by wliicli all organisms are affiliated. If, wlierever we can trace direct descent, multiplication, and divergence, this formation of groups within groups takes place; there results a strong presumption that the groups within groups which constitute the animal and vegetal kingdoms, have arisen by direct descent, multiplication, and divergence — that is, by evolu- tion.
§ 124. Strong confirmation of this inference is furnished by the fact, that the more marked differences which divide groups, are, in both cases, distinguished from the less marked differences which divide sub-groups, by this, that they are not simply greater in degree, but they are more radical in Idnd. Objects, as the stars, may present them- selves in small clusters, which are again more or less aggre- gated into clusters of clusters, in such manner that the in- dividuals of each simple cluster, are much closer together than are the simple clusters composing a compound cluster: in which case, the kinship that unites groups of groups difiers from the kinship that unites groups, not in nature, but only in amount. But this is not the case either with the groups and sub-groups which we know have resulted from evolution, or with those which we here infer have re- sulted from evolution. Among these, we find the highest or most general classes, are separated from one another by fundamental differences that have no common measure with the differences that separate small classes. Observe the pa- rallelism.
We saw that each sub-kingdom of animals is marked off from the other sub-kingdoms, by a total unlikeness in its plan of organization: that is, the members of any sub-kingdom are bound together, not by some superficial attribute which they all have, but by some attribute determining the general nature of their organizations. While, contrariwise, the members of the smallest groups are united together, and se- parated from the members of other small groups, by modi- 3G0 THE EVOLUTION OF LIFE.
iications which do not affect the essential relations of parts. That this is just the kind of arrangement which results fron evolution, the case of languages will show.
If we compare the dialects spoken in different parts of England, we find scarcely any differences but those of pro- nunciation: the structures of the sentences are abnost uniform. Between English and the allied modern languages, there are decided divergences of structure: there are some unlikenesses of idiom; some unlikenesses in the ways of modifying the meanings of verbs; and considerable unlike- nesses in the uses of genders. But these unlikenesses are not sufficient to hide a general community of organization. A greater contrast of structure exists between these modern lan- guages of Western Europe, and the classic languages. That differentiation into abstract and concrete elements, which is shown by the substitution of auxiliary words for inflections, has produced a higher specialization distinguishing these languages as a group from the older languages. JSTeverthe- less, both the ancient and modern languages of Europe, to- gether with some Eastern languages derived from the same original, have, under all their differences of organization, a fundamental community of organization; inasmuch as all of them exhibit the formation of words by such a coalescence and integration of roots as destroys the independent meanings of the roots. These Aryan languages, and others which have the amalgamate character, are united by it into a class distinguished from the aptotic and agglutinate languages; in which the roots are either not united at all, or so incompletely united that one of them still retains its independent meaning. And philologists find that these fundamental differences which severally determine the grammatical forms, or modes of com- bining ideas, are really characteristic of the primary divisions among languages.
That is to say, among languages, where we know that evolution has been going on, the gfeatest groups are marked off from one another by the strongest structural contrasts; and as the like holds among groups of organisms, there re- THE ARGUMENTS FllOM CLASSIFICATION. 301 siilts a further reason for inferring that these have been evolved.
§ 125. There is yet another parallelism of like meaning. "\Ye saw (§ 101) that the successively- subordinate classes, orders, genera, and species, into which zoologists and botan- ists segregate animals and plants, have not, in reality, those definite values conventionally given to them. There are well-marked species, and species so imperfectly defined that certain systematists regard them as varieties. Between genera, strong contrasts exist in many cases; and in other cases, contrasts so much less decided as to leave it doubtful whether they constitute generic distinctions. So, too, is it with orders and classes: in some of which there have been introduced intermediate sub -divisions, having no equivalents in others. Even of the sub- kingdoms the same truth holds. The contrast between the Molluscoida and the Molliisca, is far less than that between the Mollusca and the Annulosa; and there are naturalists who think that the Vertehrata are so much more widely separated from the other sub-kingdoms, than these are from one another, that the Vertehrata should have a classificatory value equal to that of all the other sub- kingdoms taken together.
Now just this same indefiniteness of value, or incomplete- ness of equivalence, is observable in those simple and com- pound and re-compound groups, which we see arising by evolution. In every case, the endeavour to arrange the divergent products of evolution, is met by a difiiculty like that which would meet the endeavour to classify the branches of a tree, into branches of the first, second, third, fourth, &c., orders — the difiiculty, namely, that branches of intermediate degrees of composition exist. The illustration furnished by languages will serve us once more. Some dia- lects of English are but little contrasted; others are strongly contrasted. The alliances of the several Scandinavian tongues with one another are difierent in degree. Dutch is much 362 THE EVOLUTION OF LIFE.
less distinct from German than Swedish, is; while between the Danish and Swedish there is so close a kinship, that they might almost be regarded as widely- divergent dialects. Similarly on comparing the larger divisions, we see that the various languages of the Aryan stock, have deviated from the original to very unlike distances. The general conclusion is manifest. While the kinds of hmnan speech fall into groups, and sub-groups, and sub-sub-groups; yet the groups are not equal to one another in value, nor have the sub- groups. equal values, nor the sub-sub-groups.
If, then, the classification of organisms results in several orders of assemblages, such that assemblages of the same order are but indefinitely equivalent; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly in- definite; there is additional reason for inferring that organisms are products of evolution.
§ 126. A fact of much significance remains. If groups of organic forms have arisen by divergence and re-diver- gence; and if, while the groups have been developing from simple groups into compound groups, each group and sub-group has been giving origin to more complex forms of its own type; then it is inferable that there once ex- isted greater structural likenesses between the members of allied groups, than exist now. Hence, if we take the simplest members of any group to be those which have undergone the least change; we may expect to find a greater likeness between them and the simplest members of an allied group, than we find between the more complex members of the two groups. This, speaking generally, proves to be so.
Between the sub-kingdoms, the gaps are extremely wide; but such distant kinships as may be discerned, bear out an- ticipation. Speaking of that extremely- degraded vertebrate animal the Am^hloxus, which has several molluscous traits THE ARGUMENTS FROM CLASSTFTCATTON. 363 in its organization, Dr Carpenter remarks, that it " furnislies an apt illustration of another important fact, that it is by the lowest rather than by the highest forms of two natural groups, that they are brought into closest relation." What are the faint traces of community between the Annulosa and the MoUusca? They are the thread-cells which some of their inferior grouj)s have in common with the Caelenterata. More decided approximations exist between the lower members of classes. In tracing down the Crustacea and the Arachnida from their more complex to their simpler forms, zoologists meet with difficulties: respecting some of these simpler forms, it becomes a question which class they belong to. The Lepidoswen, about which there have been disputes whether it is a fish or an amphibian, is inferior in the organization of its skeleton, to the great majority of both fishes and amphibia. Widely as they differ from them, the lower mammals have some characters in common with birds, which the higher mammals do not possess.
JN^ow since this kind of relationship of groups is not ac- counted for by any other hypothesis, while the hypothesis of evolution gives us a clue to it; we must include it among the evidences of this hypothesis, which the facts of classification furnish.^^ § 127. What shall we say of these several leading truths when taken together? That naturalists have been gradually compelled to arrange organisms in groups within groups; and that this is the arrangement which we see arises by descent, alike in individual families and among races of men, is a striking circumstance. That while the smallest groups are the most nearly related, there exist beween the great sub-kingdoms, structural contrasts of the profoundest kind; cannot but impress us as remarkable, when we see that where it is known to take place, evolution actually produces these feebly-distinguished small groups, and these strongly-dis- tinguished great groups. The impression made by these two 364 THE EVOLUTION OF LIFE.
parallelisms, which, add meaning to each other, is deepened by the third parallelism, which enforces the meaning of both — the parallelism, namely, that as, between the species, genera, orders, classes, &c., which naturalists have formed, there are transitional gradations; so between the groups, sub-groups, and sub- sub -groups, which we know to have been evolved, groups of intermediate values exist. And these three correspondences between the known results of evolution, and the results here ascribed to evolution, have further weight given to them by the circumstance, that the kinship of groups through their lowest members, is just the kinship which the hypothesis of evolution implies.
Even in the absence of these specific agreements, the broad fact of unity amid multiformity, which organisms so strik- ingly display, is strongly suggestive of evolution. Freeing ourselves from pre-conceptions, we shall see good reason to think with Mr Darwin, " 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 partly revealed to us by our classifications. '* ^^Hien we consider that this only known cause of similarity, joined with the only known cause of divergence, which we have in the influence of conditions, gives us a key to these likenesses obscured by unlikenesses, to which no consistent interpreta- tion can otherwise be given, even if purely hypothetical causes be admitted; we shall see that were there none of those very remarkable harmonies above pointed out, the truths of classification would still yield strong support to our conclusion.
CHAPTEE V.
THE ARGUMENTS FROM EMBRYOLOGY.
^ 128. There was briefly set forth in § 52, a remarkable induction established by Yon Baer; who " found that in its earliest stage, every organism has the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later, its structure is like the structures displayed at corresponding phases by a less extensive midtitude of organisms; that at each subse- quent stage, traits are acquired which successively distin- guish the developing embryo from groups of embryos that it previously resembled — thus step by step diminishing the class of embryos which it still resembles; and that thus the class of similar forms is finally narrowed to the species of which it is a member." Though this generalization is to be taken with qualifications, yet, as an average truth, it may be regarded as beyond question; and as an average truth, it has a profound significance.
For if we follow out in thought the implications of this truth — if we conceive the germs of all kinds of organisms simultaneously developing; if after taking their first step together, we imagine at the second step, one half of the vast multitude diverging from the other half; if, at the next step, we mentally watch each of these great assemblages beginning to take two or more routes of development; if we represent to ourselves this bifurcation simultaneously going on, stage after stage, in all the 366 THE EVOLUTION OF LIFE.
branches; we shall see that there must result an aggregate analogous, in its arrangement of parts, to a tree. If this vast genealogical tree be contemplated as a whole, made up of trunk, great branches, secondary branches, and so on, as far as the terminal twigs; it will be perceived that all the various kinds of organisms represented by these terminal twigs, forming the periphery of the tree, will stand related to each other in small groups, which are imited into groups of groups, and so on. The embryological tree, expressing the developmental relations of organisms, will be similar to the tree which symbolizes their classificatory relations. That subordination of classes, orders, genera, and species, to which naturalists have been gradually led, is just that subordination which results from the divergence and re-divergence of embryos, as they all unfold. On the hypothesis of evolution, this parallelism has a meaning — indicates that primordial kinship of all organisms, and that progressive differentiation of them, which the hypothesis alleges. But on any other hypothesis the parallelism is meaningless: or rather, it raises a difficulty; since it implies either an effect without a cause, or a design without a purpose.
§ 129. It was said above, that this great embryological law is to be taken with certain qualifications. The resem- blances which hold together great groups of embryos in their early stages, and which hold together smaller and smaller groups in their later and later stages, are not special or exact, but general or approximate; and in some cases, the conformity to this general law is very imperfect. These irregularities, however, instead of being at variance with the hypothesis of evolution, afford further support to it.
Observe, first, that the only two other possible suppositions respecting developmental changes, are negatived, the one by this general law and the other by the minor nonconformities to it. If it be said that the conditions of the case necessi- tated the derivation of all organisms from simple germs, and THE ARGUMENTS FROM EMBRYOLOGY. 367 tliercforc necessitated a morphological unity in their primitive states; there arises the obvious answer, that the morphologi- cal unity thus implied, is not the only morphological unity to be accounted for. Were this the only unity, the various kinds of organisms, setting out from a common primordial form, shoidd all begin from the first to diverge individually, as so many radii from a centre; which they do not. If, other- wise, it be said that organisms were framed upon certain types, and that those of the same type continue developing together in the same direction, until it is time for them to begin putting on their specialities of structure; then, the answer is, that when they do finally diverge, they ought severally to develop in direct lines towards their final forms. No reason can be assigned why, having once parted company, some should progress towards their final forms by irregular or circiutous routes. On the hypothesis of design, such de- viations are inexplicable.
The hypothesis of evolution, however, while it pre-supposes those general relations among embryos which are found to exist, also affords explanations of these minor nonconformities. If, as any rational theory of evolution pre-supposes, the pro- gressive differentiations of organic forms from one another during past times, have resulted, as they are resulting still, from the direct and indirect effects of external conditions — if organisms have become different, either by immediate adaptations to unlike habits of life, or by the mediate adapta- tions resulting from preservation of the indi\H[duals most fitted for such habits of life, or by both; and if the embryonic changes are related to the changes that were undergone by ancestral races; then these irregularities must be expected. For the successive changes in modes of life pursued by successive ancestral races, can have had no regularity of sequence. In some cases they must have been more numerous than in others; in some cases they must have been greater in degree than in others; in some cases they must have been to lower modes, in some cases to higher modes, and in some 368 THE EVOLUTION OF LIFE.
cases to modes neither higher nor lower. Of two connate races which diverged in the remote past, the one may have had descendants that have remained tolerably constant in their habits, while the other may have had descendants that have passed through widely- aberrant modes of life; and yet some of these last may have eventually taken to modes of life like those of the divergent races derived from the same stock. And if the metamorphoses of embryos, indicate, in a general way, the changes of structure undergone by ancestors; then, the later embryologic changes of such two allied races, will be somewhat different, though they may end in very similar forms. An illustration will make this clear. Mr Darwin says: — *' Petrels are the most aerial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffiniiria herardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when im- willingly it takes flight, would be mistaken by any one for an auk or grebe; nevertheless, it is essentially a petrel, but with many parts of its organization profoundly modified." Now if we suppose these grebe-like habits to be continued through a long epoch, the petrel-form to be still more obscured, and the approximation to the grebe-form still closer; it is manifest that while the chicks of the grebe and the Puffinuria will, during their early stages of development, display that likeness involved by their common derivation from some early type of bird, the chick of the Puffinuria will eventually begin to show deviations, representative of the ancestral petrel- structure, and will afterwards begin to lose these distinctions, and assume the grebe-structure.
Hence, remembering the perpetual intrusions of organisms on one another's modes of life, often widely different; and remembering that these intrusions have been going on from the beginning; we shall be prepared to find that the general law of embryologic parallelism, is qualified by irregularities that are mostly small, in many cases considerable, and THE ARGUMENTS FROM EMRRYOLOGY. 3G0 occasionally groat. The hypothesis of evolution accounts for these: it does more — it implies the necessity of them.
§ 130. The substitutions of organs and the suppressions of organs, are among those secondary embryological phe- nomena which harmonize with the belief in evolution but cannot be reconciled with any other belief. There are cases where, during its earlier stages of development, an embryo possesses organs that afterwards dwindle away, as there arise other organs to discharge the same functions. And there are cases where organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption.
We have a remarkable instance of this substitution in the successive temporary appliances for aerating the blood, which the mammalian embryo exhibits. During the first phase of its development, the mammalian embryo circulates its blood through a system of vessels distributed over what is called the area vasculosa — a system of vessels homologous with one which, among fishes, serves for aerating the blood until the permanent respiratory organs come into play. After a time, there buds out from the mammalian embryo, a vascular membrane called the allantois, homologous with one which, in birds and reptiles, replaces the first as a breathing apparatus. But while in the higher oviparous vertebrates, the allantois serves the purpose of a lung during the rest of embryonic life, it does not do so in the mamma- lian embryo. In implacental mammals, it aborts, having no function to discharge; and in the higher mammals, it becomes " placentiferous, and serves as the means of inter- communication between the parent and the offspring " — ^be- comes an organ of nutrition more than of respiration. Now since the first system of external blood-vessels, not being in contact with a directly-oxygenated medium, cannot be very serviceable to the mammalian embryo as a lung; and since 24