24 370 THE EVOLUTION OF LIFE.
tlie second system of external blood-vessels is, to the im- placental embryo, of no greater avail than tbe first; and since the communication between the embryo and the placenta among placental mammals, might as well or better have been made" directly, instead of by metamorphosis of the allantois; these substitutions appear miaccountable as results of design. But they are quite congruous with the supposition, that the mammalian type arose out of lower vertebrate types. For in such case, the mammalian embryo, passing through states representing, more or less distinctly, those which its remote ancestors had in common with the lower F^ertebrata, develops these subsidiary organs in like ways with the lower Vertehrata.
Even more striking than the substitutions of organs are the suppressions of organs. Mr Darwin names some cases as " extremely curious; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; * * * It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds." Not even temporary functions can be assigned for these organs that are first built up and then pulled down again. They are absolutely useless — their formation is absolutely superfluous. Irrecon- cilable with any teleological theory, they do not even har- monize with the theory of fixed types which are maintained by the development of all the typical parts, even where not wanted; seeing that the disappearance of these incipient organs during foetal life, spoils the typical resemblance. But while to all other hypotheses these facts are stumbling- blocks, they yield strong support to the hypothesis of evolu- tion.
Allied to these cases, are the cases of what has been called retrograde development. Many parasitic creatures and creatures which, after leading active lives for a time, eventu- ally become fixed, lose, in their adult states, the limbs and senses which they had when young. It may be alleged, THE ARGUMENTS FKOM EMBRYOLOGY. 371 however, that these creatures could not secure the habitats needful for them, without possessing during their larval stages, eyes and swimming appendages which eventually become useless; that though, by losing these, their organiza- tion retrogresses in one direction, it progresses in another direction; and that, therefore, they do not exhibit the need- less development of a higher type on the way to a lower t}^e. Nevertheless there are instances of a descent in organization, following an apparently- superfluous ascent. Mr Darwin says that in some genera of cirripedes, *^ the larva3 become developed either into hermaphrodites having the ordinary structure, or into what I have called comple- mental 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 reproduc- tion."
§ 131. Comparative embryology shows us that besides substitutions of organs, there are what may be called substi- tuted modes of development. The same kind of structure is not always produced in the same way; and some allied groups of organisms have modes of evolution which appear to be radically contrasted. The two modes are broadly dis- tinguishable as the direct and the indirect. They may severally characterize the general course of evolution as a whole, and the course of evolution in particular organs.
Thus in the immense majority of articulate animals, metamorphoses, more or less marked and more or less numerous, are passed through on the way to maturity. The familiar transformations of insects show us how circuitous is the route by which the embryo-form arrives at the adult form, among some divisions of the Articulata. But there are other divisions, as the lower Arachnida, in which the unfold- ing of the egg into the adult takes place in the simplest manner: the substance grows towards its appointed shape 372 THE EVOLUTION OF LIFE.
by the shortest route. The Mollusca furnish contrasts which, though less marked, are essentially of the same nature. Among some Gasteropods, according to Yogt, the germ-mass, after undergoing its earliest changes in the same way as germ- masses in general, begins to transform itself bodily into the finished structure: in one part, the component cells coalesce to form the heart, in another part to form the liver, and so on. But in other classes of molluscs, as the Cephalopods, the embryo is moulded out of the blastoderm, or superficial layer of the germ-mass; and the various organs, mostly aris- ing out of this blastoderm by a process of budding, reach their ultimate shapes through successive modifications, while they grow at the expense of the nutriment absorbed from the rest of the germ- mass. And this indirect development is universal among the VeTtehrata.
I Now on contemplating in their e7isem,hle, the facts thus jbriefly indicated, we may trace among these irregularities i something like a general rule. The indirect development I characterizes the most-highly- organized forms. In the sub-kingdom Vertebrata, which, considered as a whole, stands far above the rest in complexity, the development is uniformly indirect. It is indirect in the great mass of the Articulata. It is indirect in the highest Mollusca. Conversely, it is direct in a large proportion of the lower types. The eggs of Protozoa, of Coelenterata, of inferior Amiuloida, originate the respective structures proper to them, by transformations that are almost immediate; each of the cycle of forms passed through, is assumed, when the proper time comes, in the simplest way; and where they multiply by budding, the substance of the bud passes by as short a process as may be, into the finished form. Wliere among the simpler types of animals, the evolution is indirect, its indirectness generally appears to be related to some transitional mode of life, which the larva passes through on its way to maturity; and where we find direct evolution among the more complex types, it is THE ARGUMENTS FROM EMBRYOLOGY. 373 in their most degraded members: instance tlie Acari among the Articulata* We have before found that the facts of social organization, furnish us with hints to>vards interpreting the phenomena exhibited in individual organisms. Let us see whether analogies hence derived, do not help us here. A factory, or other producing establishment, or a town made up of such establishments, is an agency for elaborating some commodity consumed by society at large; and may be regarded as analogous to a gland or viscus in an individual organism. If, now, we inquire what is the primitive mode in which one of*^ these producing establishments grows up, we find it to be this. A single worker, who himself sells the produce of his labour, is the germ. His business increasing, he employs helpers — his sons or others; and having done this, he be- comes a vendor not only of his own handiwork, but of that of others. A further increase of his business compels him to multiply his assistants, and his sale grows so rapid that he is obliged to confine himself to the process of selling; that is, he ceases to be a producer, and becomes simply a channel through which the produce of others is conveyed to the public. Should his prosperity rise yet higher, he finds that he is unable to manage even the sale of his commodities, and has to employ others, probably of his own family, to aid him in selling; that is, to him as a main channel are now added subordinate channels; and so on continuously. Moreover, * It may be urged that the mode of development is obviously related to the Si2^ of the mass which is to be transformed into the embryo. Doubtless it is true that direct transformation is characteristic of small ova, and indirect trans- formation of large ova; and some such connexion may be necessary. Very pos- sibly that polarity of the physiological units, which determines the specific structure, will not act throughout a large mass in such way as to transform it bodily into the specific structure; though it Avill thus act throughout a small mass. But that the bulk of the ovum is not the sole cause of this difference of method, is proved by the fact that in some cases wliere the development is comparatively direct, as in Acteon, the ovum is very much larger than in cases where it is comparatively indirect, as in minute insects.
374 THE EVOLUTION OF LIFE.
when there grow up in one place, as a Manchester or a Birmingham, many establishments of like kind, this process is carried still further. There arise factors and agents, who are the channels through which are transmitted the pro- duce of many mills; and we believe that primarily, these factors were manufacturers who undertook to dispose of the produce of smaller houses as well as their own, and ultimately became salesmen only. Now this, which is the original mode in which social agencies of all kinds are evolved, does not continue to be the mode. There is a tendency everywhere manifested to substitute a direct process for this indirect process. Manufacturing establishments are no longer commonly developed through the series of modifica- tions above described; but mostly arise by the immediate transformation of a number of persons into master, clerks, foremen, workers, &c. Instead of business-partnerships being formed, as they originally were, by some slow unob- trusive union between traders and their sons or assistants; we now have joint- stock- companies resulting by sudden metamorphoses of groups of citizens. The like is true with larger and more complex social agencies. A new town in the United States arises not at all after the old method of gradual accumulations round a nucleus, and successive small modifications of structure accompanying increase of size; but it grows up over a large area, according to a pre- deter- mined plan; and there are developed at the outset, those various civil, ecclesiastical, and industrial centres, which the incipient city will require. Even in the formation of colonies we may similarly see, that the whole type of social organization proper to the race from which the colony comes, begins at once to show itself. There is not a gradual passing through all those developmental phases passed through by the mother- society; but there is a comparatively direct transformation of the assemblage of colonists, into a social organism allied in structure to the socia,l organism of which it was an offset.
THE ARGUMENTS FROM EMBRYOLOGY. 375 Ijet US now return to the development of individual organisms; carrying back tliis idea with us. On the hypothesis of evolution, all organs must have been originally formed after the indirect method, by the accumulation of modifications upon modifications; and if the development of the embryo repeats the development of ancestral races, organs must be thus formed in the embryo. To a consider- able extent they are thus formed. There is a striking parallelism between the mode in which, as above described, manufacturing agencies are originally evolved, and the mode in which secreting organs are evolved. Out of the group of bile- cells forming the germ of the liver, some centrally-placed ones, lying next to the intestine, are trans- formed into ducts through which the secretion of the peri- pheral bile-cells is poured into the intestine; and as the peri- pheral bile-cells multiply, there similarly arise secondary ducts emptying themselves into the main ones; tertiary ones into these; and so on. But while in this and in other organs, the development remains in a great degree indirect; there are organs, as the heart, in which it is comparatively direct. The heart of the vertebrate embryo does not arise from a bud; but it is first traceable as an aggregated mass of cells, becoming distinct from the cells amid which it is imbedded: its transformation into a contractile chamber, is efiected by the consolidation of its outer cells while its inner cells liquify. And the comparatively direct development thus displayed in some organs of the higher embryos, is, as we have seen, characteristic of the entire development in many lower embryos.
On the hypothesis of evolution, the direct mode of de- velopment in animals, must have been substituted for the indirect mode; as we see that it is substituted in societies. How comes it to have been substituted? By studying the cause of the substitution in the social organism, we may perhaps get some insight into its cause in the indi^T^dual or- ganism. The direct mode of forming social agencies 376 THE EVOLUTION OF LIFE.
replaces tlie indirect mode, when these social agencies have either heen so long established, or have become so prevalent, or both, as to modify the people's habits and ideas. Groups of citizens unite into corporate bodies which quickly organ- ize, because the habit of forming such combinations has so far modified the thoughts and feelings of citizens, that it becomes natural to them thus to arrange themselves. So, too, is it with the men who form a colony. The rapid as- sumption by them of a social structure, as similar as circum- stances permit to the structure of the mother- society, is manifestly due to the fact, that the organization of the mother-society has moulded the emotions and beliefs of its members into conformity with itself; so that when some of its members are transferred to a colony, they arrange themselves directly into a structure of like type with that of the mother-society: they do not repeat all the stages through which the mother- society passed, because their natures have been too far modified to allow of their doing this. That action and reaction between a social organism and its units, which we here see accounts for changes in modes of social development, must be paralleled by the action and reaction between an individual organism and its units. Various classes of phenomena compelled us to conclude, that each kind of organism is composed of physiological units, having certain peculiarites which force them to arrange themselves into the form of the species to which they are peculiar. And in the chapters on Genesis, Heredity, and Variation, we saw reason to believe, that while the polarities of the physiological units determine the structure of the organism as a whole; the organism as a whole, if its structure is changed by incident forces, reacts on the physiological units, and modifies them towards con- formity with its new structure. Now this action and reac- tion between an organic aggregate and its units, tending ever to bring the two into absolute harmony, must be con- tinually making the developmental processes more direct; THE ARGUMENTS FROM EMBRYOLOGY. 377 and will show its effects in all kinds of ways and degrees, according to tlie ancestral history of each species. Suppos- ing it were possible for a race of organisms to have con- tinued propagating itself through an indefinitely-long period without any change of conditions, necessitating change of structure; there would be reached so complete a congruity between the organic aggregate and its physiologi- cal units, that the units would arrange themselves directly into a structure like that of the adult organism: the germ would put on the proper characters of the species, with little or no transposition of substance. But in the absence of any such constancy of conditions and structure, what may we expect? We may expect that where the conditions and structure have been most constant, the mode of develop- ment will be the most direct; and that it will be the most indirect, where there have been the greatest and most numerous changes in the habits and structures of ancestral races of organisms. And we may also expect that develop- mental changes corresponding to early ancestral forms, will undergo an obliteration that is great in proportion to the fixity of organization that has been since maintained. The facts appear in harmony with this conclusion. We see a comparatively- direct development in those inferior types of animals, which show us, by their inferiority, that they have not, since the commencement of organic life, passed through many sets of changes. And where we find direct de- velopment among higher types of animals, it characterizes the simpler rather than the more complex members of the types.
Between different parts in the same embryo, there are un- likenesses in the method of formation, which seem to have kindred meanings. The heart, of which the development is in great measure direct, is an organ that appears compara- tively early among the ascending grades of organic forms; and having appeared, retains throughout the character of a hollow muscle. Conversely, the organs which develop with 378 THE EVOLUTION OF LIFE.
great indirectness, are the organs of external relation; which, in the progress of organic forms, undergo various metamorphoses. Some light, too, is thus throT\Ti on certain irregularities in the order of development of organs. If we contemplate those continuous actions and reactions which tend ever to establish a balance between an organic aggregate and its units; we shall see that the effect which the units composing any organ, produce on the organism as a whole, will depend, partly on the permanence of such organ, and partly on its proportional mass. The influence of any force, is a product of its amount multiplied into the time during which it has acted. Hence, a larger part of the aggregate acting for a shorter time, will impress itself on the phy- siological units, as much as a smaller part acting for a longer time; and may thus begin to show its influence in the developmental changes, as soon as, or even earlier than, a part that has existed for a greater period. Thus it becomes comprehensible why, in certain Entozoa which have im- mensely-developed generative systems, the rudiments of the generative systems are the first to become visible. And thus are also explicable, anomalies such as those pointed out by Prof. Agassiz — the appearance, in some cases, of traits characterizing the species, at an earlier period of development than traits characterizing the genus.
§ 132. So that while the embryologic law enunciated by Von Baer, is in harmony with the hypothesis of evolution, and is, indeed, a law which this hypothesis implies; the minor nonconformities to the law, are also interpretable by this hypothesis. Parallelism between the courses of develop- ment in species that had a common ancestry, is liable to be variously modified in correspondence with the later ancestral forms passed through after divergence of such species. The substitution of a direct for an indirect process of formation, which we have reason to believe will show itself, both in the unfolding of the entire organism and in the unfolding of par- 1 THE ARGUMENTS FROM EMBRYOLOGY. 379 ticular organs, must obscure the cmbryologic history. And the parts influencing the whole in degrees varying with their masses, there results a further influence which, from the out- set, must begin to modify the metamorphoses of each kind of embryo; and cause it to show incipient divergences from embryos which had ancestral histories the same as its own. Thus we find three different causes conspiring in endless ways and degrees, to produce deviations from the general law — causes which are manifestly capable of producing, under special conditions, changes in apparent contradiction to this law.
CIIAPTEE YI.
THE ARGUMENTS FROM MORPHOLOGY.
§ 133. Leaving out of consideration the parallelism of development which, characterizes organisms belonging to each group, that community of plan which exists among them when they are mature, is extremely remarkable and extremely suggestive. As before shown (§ 103), neither the supposition that these combinations of attributes which unite classes are fortuitous, nor the supposition that no other combinations were practicable, nor the supposition of adherence to pre- determined typical plans, suffices to explain the facts. An instance will best prepare the reader for seeing the true meaning of these fundamental likenesses.
Under the immensely- varied forms of insects, greatly elon- gated like the dragon-fly, or contracted in shape like the lady-bird, winged lilvc the butterfly, or wingless like the flea, we find this character in common — there are primarily twenty segments. These segments may be distinctly marked, or they may be so fused as to make it difficult to find the divisions between them. This is not all. It has been shown that the same number of segments is possessed by all the Crustacea, The highly- consolidated crab, and the squilla with its long, loosely-jointed divisions, are composed of the same number of somites. Though, in the higher crustaceans, some of these successive indurated rings, forming the exo- skeleton, are never more than partially marked ofi" from each THE ARGUMENTS FROM MORrHOLOGY. 381 otlicr; yet they are iudcntifiable as homologous with segments, which, in other crustaceans, are definitely divided. What, now, can be the meaning of this community of structure among these hundreds of thousands of S23ecies filling the air, burrowing in the earth, swimming in the water, creeping about among the sea- weed, and having such enormous differences of size, outline, and substance, as that no community would be suspected between them? Why under the down-covered body of the moth and under the hard wing-cases of the beetle, should there be discovered the same number of divisions as in the calcareous framework of the lobster? It cannot be by chance that there exist just twenty segments in all these hundreds of thousands of species. There is no reason to think it was necessary, in the sense that no other number would have made a possible organism. And to say that it is the result of design — to say that the Cre- ator followed this pattern throughout, merely for the purpose of maintaining the pattern — ^is to assign a motive which, if avowed by a human being, we should call whimsical. JSTo rational interpretation of this and hosts of like morphological truths, can be given except by the hypothesis of evolution; and from the hypothesis of evolution they are corollaries. If organic forms have arisen from common stocks by per- petual divergences and redivergences — if they have continued to inherit, more or less clearly, the characters of ancestral races; then there will naturally result these communities of fundamental structure among extensive assemblages of crea- tures, that have severally become modified in countless ways and degrees, in adaptation to their respective modes of life. To this let it be added, that while the belief in an intentional adhesion to a pre-determined pattern throughout a whole group, is totally negatived by the occur- rence of occasional deviations from the pattern; such devi- ations are reconcilable with the belief in evolution. As pointed out in the last chapter, there is reason to think that remote ancestral traits, will be obscured more or less according 382 THE EVOLUTION OF LIFE.
as the superposed modifications of structure, have or have not been great or long maintained. Hence, though the occur- rence of articulate animals, such as spiders and mites, having fewer than twenty segments, is fatal to the supposition that twenty segments was decided on for the three groups of superior Articulata; it is not incongruous with the supposition, that some primitive race of articulate animals, bequeathed to these three groups this common typical character — a character which has nevertheless, in many cases, become greatly ob- scured, and in some of the most aberrant orders of these classes, quite lost.
§ 134. Besides these wide-embracing and often deeply- hidden homologies, which hold together different animals, there are the scarcely-less significant homologies between different organs of the same animal. These homologies, like the others, are obstacles to the supernatural interpreta- tions, and supports of the natural interpretation.
One of the most familiar and instructive instances is furnished by the vertebral column. Snakes, which move sinuously through and over plants and stones, obviously need a segmentation of the bony axis from end to end; and inasmuch as flexibility is required throughout the whole length of the body, there is advantage in the comparative uniformity of this segmentation: the creature's movements would be impeded if, instead of a chain of vertebrae varying but little in their lengths, there existed in the middle of the series some long bony mass that would not bend. But in most of the higher Vertehratay the mechanical actions and reac- tions demand that while some parts of the vertebral axis shall be flexible, other parts shall be inflexible. Inflexibility is especially requisite in that part of the vertebral column called the sacrum; which, in mammals and birds, forms a fulcrum exposed to the greatest strains which the skeleton has to bear. Now in both mammals and birds, this rigid portion of the vertebral column is not made of one long THE argump:^ts from morphology. 383