Whenever the remains of bats have been found they have presented the exact type of existing forms, and there is as yet no indication of the conditions of an incipient elevation from the ground.
The pterodactyles, again, though a numerous group, are all true and perfect pterodactyles, though surely some of the many incipient forms, which on the Darwinian theory have existed, must have had a good chance of preservation.
As to birds, the only notable instance in which discoveries recently made appear to fill up an important hiatus, is the interpretation given by Professor Huxley[127] to the remains of Dinosaurian reptiles, and which were noticed in the third chapter of this work. The learned Professor has (as also has Professor Cope in America) shown that in very important and significant points the skeletons of the Iguanodon and of its allies approach very closely to that existing in the ostrich, emeu, rhea, &c. He has given weighty reasons for thinking that the line of affinity between birds and reptiles passes to the birds last named from the Dinosauria rather than from the Pterodactyles, through Archeopteryx-like forms to the ordinary birds. Finally, he has thrown out the suggestion that the celebrated footsteps left by some extinct three-toed creatures on the very ancient sandstone of Connecticut were made, not, as hitherto supposed, by true birds, but by more or less ornithic reptiles. But even supposing all that is asserted or inferred on this subject to be fully proved, it would not approach to a demonstration of specific origin by minute modification. And though it harmonizes well with "Natural Selection," it is equally consistent with the rapid and sudden development of new specific forms of life. Indeed, Professor Huxley, with a laudable caution and moderation too little observed by some Teutonic Darwinians, guarded himself carefully from any imputation of asserting dogmatically the theory of "Natural Selection," while upholding fully the doctrine of evolution.
But, after all, it is by no means certain, though very probable, that the Connecticut footsteps were made by very ornithic reptiles, or extremely sauroid birds. And it must not be forgotten that a completely carinate[128] bird (the Archeopteryx) existed at a time, when, as yet, we have no evidence of some of the Dinosauria having come into being. Moreover, if the remarkable and minute similarity of the coracoid of a pterodactyle to that of a bird be merely the result of function and no sign of genetic affinity, it is not inconceivable that pelvic and leg resemblances of Dinosauria to birds may be functional likewise, though such an explanation is, of course, by no means necessary to support the view maintained in this book.
But the number of forms represented by many individuals, yet by no transitional ones, is so great that only two or three can be selected as examples. Thus those remarkable fossil reptiles, the Ichthyosauria and Plesiosauria, extended, through the secondary period, probably over the greater part of the globe. Yet no single transitional form has yet been met with in spite of the multitudinous individuals preserved. Again, with their modern representatives the Cetacea, one or two aberrant forms alone have been found, but no series of transitional ones indicating minutely the line of descent. This group, the whales, is a very marked one, and it is curious, on Darwinian principles, that so few instances tending to indicate its mode of origin should have presented themselves. Here, as in the bats, we might surely expect that some relics of unquestionably incipient stages of its development would have been left.
The singular order Chelonia, including the tortoises, turtles, and terrapins (or fresh-water tortoises), is another instance of an extreme form without any, as yet known, transitional stages. Another group may be finally mentioned, viz. the frogs and toads, anourous Batrachians, of which we have at present no relic of any kind linking them on to the Eft group on the one hand, or to reptiles on the other.
The only instance in which an approach towards a series of nearly related forms has been obtained is the existing horse, its predecessor Hipparion and other extinct forms. But even here there is no proof whatever of modification by minute and infinitesimal steps; a fortiori no approach to a proof of modification by "Natural Selection," acting upon indefinite fortuitous variations. On the contrary, the series is an admirable example of successive modification in one special direction along one beneficial line, and the teleologist must here be allowed to consider that one motive of this modification (among probably an indefinite number of motives inconceivable to us) was the relationship in which the horse was to stand to the human inhabitants of this planet. These extinct forms, as Professor Owen, remarks,[129] "differ from each other in a greater degree than do the horse, zebra, and ass," which are not only good zoological species as to form, but are species physiologically, i.e. they cannot produce a race of hybrids fertile inter se.
As to the mere action of surrounding conditions, the same Professor remarks:[130] "Any modification affecting the density of the soil might so far relate to the changes of limb-structure, as that a foot with a pair of small hoofs dangling by the sides of the large one, like those behind the cloven hoof of the ox, would cause the foot of Hipparion, e.g., and a fortiori the broader based three-hoofed foot of the Palæothere, to sink less deeply into swampy soil, and be more easily withdrawn than the more concentratively simplified and specialized foot of the horse. Rhinoceroses and zebras, however, tread together the arid plains of Africa in the present day; and the horse has multiplied in that half of America where two or more kinds of tapir still exist. That the continents of the Eocene or Miocene periods were less diversified in respect of swamp and sward, pampas or desert, than those of the Pliocene period, has no support from observation or analogy."
Not only, however, do we fail to find any traces of the incipient stages of numerous very peculiar groups of animals, but it is undeniable that there are instances which appeared at first to indicate a gradual transition, yet which instances have been shown by further investigation and discovery not to indicate truly anything of the kind. Thus at one time the remains of Labyrinthodonts, which up till then had been discovered, seemed to justify the opinion that as time went on, forms had successively appeared with more and more complete segmentation and ossification of the backbone, which in the earliest forms was (as it is in the lowest fishes now) a soft continuous rod or notochord. Now, however, it is considered probable that the soft back-boned Labyrinthodont Archegosaurus, was an immature or larval form,[131] while Labyrinthodonts with completely developed vertebræ have been found to exist amongst the very earliest forms yet discovered. The same may be said regarding the eyes of the trilobites, some of the oldest forms having been found as well furnished in that respect as the very last of the group which has left its remains accessible to observation.
Such instances, however, as well as the way in which marked and special forms (as the Pterodactyles, &c., before referred to) appear at once in and similarly disappear from the geological record, are of course explicable on the Darwinian theory, provided a sufficiently enormous amount of past time be allowed. The alleged extreme, and probably great, imperfection of that record may indeed be pleaded in excuse. But it is an excuse.[132] Nor is it possible to deny the a priori probability of the preservation of at least a few minutely transitional forms in some instances if every species without exception has arisen exclusively by such minute and gradual transitions.
It remains, then, to turn to the other considerations with regard to the relation of species to time: namely (1) as to the total amount of time allowable by other sciences for organic evolution; and (2) the proportion existing, on Darwinian principles, between the time anterior to the earlier fossils, and the time since; as evidenced by the proportion between the amount of evolutionary change during the latter epoch and that which must have occurred anteriorly.
Sir William Thomson has lately[133] advanced arguments from three distinct lines of inquiry, and agreeing in one approximate result. The three lines of inquiry were—1. The action of the tides upon the earth's rotation. 2. The probable length of time during which the sun has illuminated this planet; and 3. The temperature of the interior of the earth. The result arrived at by these investigations is a conclusion that the existing state of things on the earth, life on the earth, all geological history showing continuity of life, must be limited within some such period of past time as one hundred million years. The first question which suggests itself, supposing Sir W. Thomson's views to be correct, is, Is this period anything like enough for the evolution of all organic forms by "Natural Selection"? The second is, Is this period anything like enough for the deposition of the strata which must have been deposited if all organic forms have been evolved by minute steps, according to the Darwinian theory?
In the first place, as to Sir William Thomson's views, the Author of this book cannot presume to advance any opinion; but the fact that they have not been refuted, pleads strongly in their favour when we consider how much they tell against the theory of Mr. Darwin. The last-named author only remarks that "many of the elements in the calculation are more or less doubtful,"[134] and Professor Huxley[135] does not attempt to refute Sir W. Thomson's arguments, but only to show cause for suspense of judgment, inasmuch as the facts may be capable of other explanations.
Mr. Wallace, on the other hand,[136] seems more disposed to accept them, and, after considering Sir William's objections and those of Mr. Croll, puts the probable date of the beginning of the Cambrian deposits[137] at only twenty-four million years ago. On the other hand, he seems to consider that specific change has been more rapid than generally supposed, and exceptionally stable during the last score or so of thousand years.
Now, first, with regard to the time required for the evolution of all organic forms by merely accidental, minute, and fortuitous variations, the useful ones of which have been preserved:
Mr. Murphy[138] is distinctly of opinion that there has not been time enough. He says, "I am inclined to think that geological time is too short for the evolution of the higher forms of life out of the lower by that accumulation of imperceptibly slow variations, to which alone Darwin ascribes the whole process."
"Darwin justly mentions the greyhound as being equal to any natural species in the perfect co-ordination of its parts, 'all adapted for extreme fleetness and for running down weak prey.'" "Yet it is an artificial species (and not physiologically a species at all), formed by long-continued selection under domestication; and there is no reason to suppose that any of the variations which have been selected to form it have been other than gradual and almost imperceptible. Suppose that it has taken five hundred years to form the greyhound out of his wolf-like ancestor. This is a mere guess, but it gives the order of the magnitude." Now, if so, "how long would it take to obtain an elephant from a protozoon, or even from a tadpole-like fish? Ought it not to take much more than a million times as long?"[139]
Mr. Darwin[140] would compare with the natural origin of a species "unconscious selection, that is, the preservation of the most useful or beautiful animals, with no intention of modifying the breed." He adds: "But by this process of unconscious selection, various breeds have been sensibly changed in the course of two or three centuries."
"Sensibly changed!" but not formed into "new species." Mr. Darwin, of course, could not mean that species generally change so rapidly, which would be strangely at variance with the abundant evidence we have of the stability of animal forms as represented on Egyptian monuments and as shown by recent deposits. Indeed, he goes on to say,—"Species, however, probably change much more slowly, and within the same country only a few change at the same time. This slowness follows from all the inhabitants of the same country being already so well adapted to each other, that places in the polity of nature do not occur until after long intervals, when changes of some kind in the physical conditions, or through immigration, have occurred, and individual differences and variations of the right nature, by which some of the inhabitants might be better fitted to their new places under altered circumstances, might not at once occur." This is true, and not only will these changes occur at distant intervals, but it must be borne in mind that in tracing back an animal to a remote ancestry, we pass through modifications of such rapidly increasing number and importance that a geometrical progression can alone indicate the increase of periods which such profound alterations would require for their evolution through "Natural Selection" only.
Thus let us take for an example the proboscis monkey of Borneo (Semnopithecus nasalis). According to Mr. Darwin's own opinion, this form might have been "sensibly changed" in the course of two or three centuries. According to this, to evolve it as a true and perfect species one thousand years would be a very moderate period. Let ten thousand years be taken to represent approximately the period of substantially constant conditions during which no considerable change would be brought about. Now, if one thousand years may represent the period required for the evolution of the species S. nasalis, and of the other species of the genus Semnopithecus; ten times that period should, I think, be allowed for the differentiation of that genus, the African Cercopithecus and the other genera of the family Simiidæ—the differences between the genera being certainly more than tenfold greater than those between the species of the same genus. Again we may perhaps interpose a period of ten thousand years' comparative repose.
For the differentiation of the families Simiidæ and Cebidæ—so very much more distinct and different than any two genera of either family—a period ten times greater should, I believe, be allowed than that required for the evolution of the subordinate groups. A similarly increasing ratio should be granted for the successive developments of the difference between the Lemuroid and the higher forms of primates; for those between the original primate and other root-forms of placental mammals; for those between primary placental and implacental mammals, and perhaps also for the divergence of the most ancient stock of these and of the monotremes, for in all these cases modifications of structure appear to increase in complexity in at least that ratio. Finally, a vast period must be granted for the development of the lowest mammalian type from the primitive stock of the whole vertebrate sub-kingdom. Supposing this primitive stock to have arisen directly from a very lowly organized animal indeed (such as a nematoid worm, or an ascidian, or a jelly-fish), yet it is not easy to believe that less than two thousand million years would be required for the totality of animal development by no other means than minute, fortuitous, occasional, and intermitting variations in all conceivable directions. If this be even an approximation to the truth, then there seem to be strong reasons for believing that geological time is not sufficient for such a process.
The second question is, whether there has been time enough for the deposition of the strata which must have been deposited, if all organic forms have been evolved according to the Darwinian theory?
Now this may at first seem a question for geologists only, but, in fact, in this matter geology must in some respects rather take its time from zoology than the reverse; for if Mr. Darwin's theory be true, past time down to the deposition of the Upper Silurian strata can have been but a very small fraction of that during which strata have been deposited. For when those Upper Silurian strata were formed, organic evolution had already run a great part of its course, perhaps the longest, slowest, and most difficult part of that course.
At that ancient epoch not only were the vertebrate, molluscous, and arthropod types distinctly and clearly differentiated, but highly developed forms had been produced in each of these sub-kingdoms. Thus in the Vertebrata there were fishes not belonging to the lowest but to the very highest groups which are known to have ever been developed, namely, the Elasmobranchs (the highly organized sharks and rays) and the Ganoids, a group now poorly represented, but for which the sturgeon may stand as a type, and which in many important respects more nearly resemble higher Vertebrata than do the ordinary or osseous fishes. Fishes in which the ventral fins are placed in front of the pectoral ones (i.e. jugular fishes) have been generally considered to be comparatively modern forms. But Professor Huxley has kindly informed me that he has discovered a jugular fish in the Permian deposits.
Amongst the molluscous animals we have members of the very highest known class, namely, the Cephalopods, or cuttle-fish class; and amongst articulated animals we find Trilobites and Eurypterida, which do not belong to any incipient worm-like group, but are distinctly differentiated Crustacea of no low form.
We have in all these animal types nervous systems differentiated on distinctly different patterns, fully formed organs of circulation, digestion, excretion, and generation, complexly constructed eyes and other sense organs; in fact, all the most elaborate and complete animal structures built up, and not only once, for in the fishes and mollusca we have (as described in the third chapter of this work) the coincidence of the independently developed organs of sense attaining a nearly similar complexity in two quite distinct forms. If, then, so small an advance has been made in fishes, molluscs, and arthropods since the Upper Silurian deposits, it will probably be within the mark to consider that the period before those deposits (during which all these organs would, on the Darwinian theory, have slowly built up their different perfections and complexities) occupied time at least a hundredfold greater.
Now it will be a moderate computation to allow 25,000,000 years for the deposition of the strata down to and including the Upper Silurian. If, then, the evolutionary work done during this deposition, only represents a hundredth part of the sum total, we shall require 2,500,000,000 (two thousand five hundred million) years for the complete development of the whole animal kingdom to its present state. Even one quarter of this, however, would far exceed the time which physics and astronomy seem able to allow for the completion of the process.
Finally, a difficulty exists as to the reason of the absence of rich fossiliferous deposits in the oldest strata—if life was then as abundant and varied as, on the Darwinian theory, it must have been. Mr. Darwin himself admits[141] "the case at present must remain inexplicable; and may be truly urged as a valid argument against the views" entertained in his book.
Thus, then, we find a wonderful (and on Darwinian principles an all but inexplicable) absence of minutely transitional forms. All the most marked groups, bats, pterodactyles, chelonians, ichthyosauria, anoura, &c., appear at once upon the scene. Even the horse, the animal whose pedigree has been probably best preserved, affords no conclusive evidence of specific origin by infinitesimal, fortuitous variations; while some forms, as the labyrinthodonts and trilobites, which seemed to exhibit gradual change, are shown by further investigation to do nothing of the sort. As regards the time required for evolution (whether estimated by the probably minimum period required for organic change or for the deposition of strata which accompanied that change), reasons have been suggested why it is likely that the past history of the earth does not supply us with enough. First, because of the prodigious increase in the importance and number of differences and modifications which we meet with as we traverse successively greater and more primary zoological groups; and, secondly, because of the vast series of strata necessarily deposited if the period since the Lower Silurian marks but a small fraction of the period of organic evolution. Finally, the absence or rarity of fossils in the oldest rocks is a point at present inexplicable, and not to be forgotten or neglected.
Now all these difficulties are avoided if we admit that new forms of animal life of all degrees of complexity appear from time to time with comparative suddenness, being evolved according to laws in part depending on surrounding conditions, in part internal—similar to the way in which crystals (and, perhaps from recent researches, the lowest forms of life) build themselves up according to the internal laws of their component substance, and in harmony and correspondence with all environing influences and conditions.
CHAPTER VII.
SPECIES AND SPACE.
The geographical distribution of animals presents difficulties.—These not insurmountable in themselves; harmonize with other difficulties.—Fresh-water fishes.—Forms common to Africa and India; to Africa and South America; to China and Australia; to North America and China; to New Zealand and South America; to South America and Tasmania; to South America and Australia.—Pleurodont lizards.—Insectivorous mammals.—Similarity of European and South American frogs—Analogy between European salmon and fishes of New Zealand, &c. An ancient Antarctic continent probable.—Other modes of accounting for facts of distribution.—Independent origin of closely similar forms.—Conclusion.
The study of the distribution of animals over the earth's surface presents us with many facts having certain not unimportant bearings on the question of specific origin. Amongst these are instances which, at least at first sight, appear to conflict with the Darwinian theory of "Natural Selection." It is not, however, here contended that such facts do by any means constitute by themselves obstacles which cannot be got over. Indeed it would be difficult to imagine any obstacles of the kind which could not be surmounted by an indefinite number of terrestrial modifications of surface—submergences and emergences—junctions and separations of continents in all directions and combinations of any desired degree of frequency. All this being supplemented by the intercalation of armies of enemies, multitudes of ancestors of all kinds, and myriads of connecting forms, whose raison d'être may be simply their utility or necessity for the support of the theory of "Natural Selection."
Nevertheless, when brought in merely to supplement and accentuate considerations and arguments derived from other sources, in that case difficulties connected with the geographical distribution of animals are not without significance, and are worthy of mention even though, by themselves, they constitute but feeble and more or less easily explicable puzzles which could not alone suffice either to sustain or to defeat any theory of specific origination.
Many facts as to the present distribution of animal life over the world are very readily explicable by the hypothesis of slight elevations and depressions of larger and smaller parts of its surface, but there are others the existence of which it is much more difficult so to explain.
The distribution either of animals possessing the power of flight, or of inhabitants of the ocean, is, of course, easily to be accounted for; the difficulty, if there is really any, must mainly be with strictly terrestrial animals of moderate or small powers of locomotion and with inhabitants of fresh water. Mr. Darwin himself observes,[142] "In regard to fish, I believe that the same species never occur in the fresh waters of distant continents." Now, the Author is enabled, by the labours and through the kindness of Dr. Günther, to show that this belief cannot be maintained; he having been so obliging as to call attention to the following facts with regard to fish-distribution. These facts show that though only one species which is absolutely and exclusively an inhabitant of fresh water is as yet known to be found in distant continents, yet that in several other instances the same species is found in the fresh water of distant continents, and that very often the same genus is so distributed.
The genus Mastacembelus belongs to a family of fresh-water Indian fishes. Eight species of this genus are described by Dr. Günther in his catalogue.[143] These forms extend from Java and Borneo on the one hand, to Aleppo on the other. Nevertheless, a new species (M. cryptacanthus) has been described by the same author,[144] which is an inhabitant of the Camaroon country of Western Africa. He observes, "The occurrence of Indian forms on the West Coast of Africa, such as Periophthalmus, Psettus, Mastacembelus, is of the highest interest, and an almost new fact in our knowledge of the geographical distribution of fishes."
Ophiocephalus, again, is a truly Indian genus, there being no less than twenty-five species,[145] all from the fresh waters of the East Indies. Yet Dr. Günther informs me that there is a species in the Upper Nile and in West Africa.
The acanthopterygian family (Labyrinthici) contains nine freshwater genera, and these are distributed between the East Indies and South and Central Africa.
The Carp fishes (Cyprinoids) are found in India, Africa, and Madagascar, but there are none in South America.
Thus existing fresh-water fishes point to an immediate connexion between Africa and India, harmonizing with what we learn from Miocene mammalian remains.
On the other hand, the Characinidæ (a family of the physostomous fishes) are found in Africa and South America, and not in India, and even its component groups are so distributed,—namely, the Tetragonopterina[146] and the Hydrocyonina.[147]
Again, we have similar phenomena in that almost exclusively fresh-water group the Siluroids.
Thus the genera Clarias[148] and Heterobranchus[149] are found both in Africa and the East Indies. Plotosus is found in Africa, India, and Australia, and the species P. anguillaris[150] has been brought from both China and Moreton Bay. Here, therefore, we have the same species in two distinct geographical regions. It is however a coast fish, which, though entering rivers, yet lives in the sea.
Eutropius[151] is an African genus, but E. obtusirostris comes from India. On the other hand, Amiurus is a North American form; but one species, A. cantonensis,[152] comes from China.
The genus Galaxias[153] has at least one species common to New Zealand and South America, and one common to South America and Tasmania. In this genus we thus have an absolutely and completely fresh-water form of the very same species distributed between different and distinct geographical regions.
Of the lower fishes, a lamprey, Mordacia mordax,[154] is common to South Australia and Chile; while another form of the same family, namely, Geotria chilensis,[155] is found not only in South America and Australia, but in New Zealand also. These fishes, however, probably pass part of their lives in the sea.
We thus certainly have several species which are common to the fresh waters of distant continents, although it cannot be certainly affirmed that they are exclusively and entirely fresh-water fishes throughout all their lives except in the case of Galaxias.
Existing forms point to a close union between South America and Africa on the one hand, and between South America, Australia, Tasmania, and New Zealand on the other; but these unions were not synchronous any more than the unions indicated between India and Australia, China and Australia, China and North America, and India and Africa.
Pleurodont lizards are such as have the teeth attached by their sides to the inner surface of the jaw, in contradistinction to acrodont lizards, which have the bases of their teeth anchylosed to the summit of the margin of the jaw. Now pleurodont iguanian lizards abound in the South American region; but nowhere else, and are not as yet known to inhabit any part of the present continent of Africa. Yet pleurodont lizards, strange to say, are found in Madagascar. This is the more remarkable, inasmuch as we have no evidence yet of the existence in Madagascar of fresh-water fishes common to Africa and South America.
(Showing the teeth attached to the inner surface of its side.)
Again, that remarkable island Madagascar is the home of very singular and special insectivorous beasts of the genera Centetes, Ericulus, and Echinops; while the only other member of the group to which they belong is Solenodon, which is a resident in the West Indian Islands, Cuba and Hayti. The connexion, however, between the West Indies and Madagascar must surely have been at a time when the great lemurine group was absent; for it is difficult to understand the spread of such a form as Solenodon, and at the same time the non-extension of the active lemurs, or their utter extirpation, in such a congenial locality as the West Indian Archipelago.
The close connexion of South America and Australia is demonstrated (on the Darwinian theory), not only from the marsupial fauna of both, but also from the frogs and toads which respectively inhabit those regions. A truly remarkable similarity and parallelism exist, however, between certain of the same animals inhabiting South Western America and Europe. Thus Dr. Günther has described[156] a frog from Chile by the name of cacotus, which singularly resembles the European bombinator.
Again of the salmons, two genera from South America, New Zealand, and Australia, are analogous to European salmons.
In addition to this may be mentioned a quotation from Professor Dana, given by Mr. Darwin,[157] to the effect that "it is certainly a wonderful fact that New Zealand should have a closer resemblance in its crustacea to Great Britain, its antipode, than to any other part of the world:" and Mr. Darwin adds "Sir J. Richardson also speaks of the reappearance on the shores of New Zealand, Tasmania, &c. of northern forms of fish. Dr. Hooker informs me that twenty-five species of algæ are common to New Zealand and to Europe, but have not been found in the intermediate tropical seas."
Many more examples of the kind could easily be brought, but these must suffice. As to the last-mentioned cases Mr. Darwin explains them by the influence of the glacial epoch, which he would extend actually across the equator, and thus account, amongst other things, for the appearance in Chile of frogs having close genetic relations with European forms. But it is difficult to understand the persistence and preservation of such exceptional forms with the extirpation of all the others which probably accompanied them, if so great a migration of northern kinds had been occasioned by the glacial epoch.
Mr. Darwin candidly says,[158] "I am far from supposing that all difficulties in regard to the distribution and affinities of the identical and allied species, which now live so widely separated in the north and south, and sometimes on the intermediate mountain-ranges, are removed." ... "We cannot say why certain species and not others have migrated; why certain species have been modified and have given rise to new forms, whilst others have remained unaltered." Again he adds, "Various difficulties also remain to be solved; for instance, the occurrence, as shown by Dr. Hooker, of the same plants at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs, as suggested by Lyell, may have been concerned in their dispersal. The existence, at these and other distant points of the southern hemisphere, of species which, though distinct, belong to genera exclusively confined to the south, is a more remarkable case. Some of these species are so distinct that we cannot suppose that there has been time since the commencement of the last glacial period for their migration and subsequent modification to the necessary degree." Mr. Darwin goes on to account for these facts by the probable existence of a rich antarctic flora in a warm period anterior to the last glacial epoch. There are indeed many reasons for thinking that a southern continent, rich in living forms, once existed. One such reason is the way in which struthious birds are, or have been, distributed around the antarctic region: as the ostrich in Africa, the rhea in South America, the emeu in Australia, the apteryx, dinornis, &c. in New Zealand, the epiornis in Madagascar. Still the existence of such a land would not alone explain the various geographical cross relations which have been given above. It would not, for example, account for the resemblance between the crustacea or fishes of New Zealand and of England. It would, however, go far to explain the identity (specific or generic) between fresh water and other forms now simultaneously existing in Australia and South America, or in either or both of these, and New Zealand.
Again, mutations of elevation small and gradual (but frequent and intermitting), through enormous periods of time—waves, as it were, of land rolling many times in many directions—might be made to explain many difficulties as to geographical distribution, and any cases that remained would probably be capable of explanation, as being isolated but allied animal forms, now separated indeed, but being merely remnants of extensive groups which, at an earlier period, were spread over the surface of the earth. Thus none of the facts here given are any serious difficulty to the doctrine of "evolution," but it is contended in this book that if other considerations render it improbable that the manifestation of the successive forms of life has been brought about by minute, indefinite, and fortuitous variations, then these facts as to geographical distribution intensify that improbability, and are so far worthy of attention.
All geographical difficulties of the kind would be evaded if we could concede the probability of the independent origin, in different localities, of the same organic forms in animals high in the scale of nature. Similar causes must produce similar results, and new reasons have been lately adduced for believing, as regards the lowest organisms, that the same forms can arise and manifest themselves independently. The difficulty as to higher animals is, however, much greater, as (on the theory of evolution) one acting force must always be the ancestral history in each case, and this force must always tend to go on acting in the same groove and direction in the future as it has in the past. So that it is difficult to conceive that individuals, the ancestral history of which is very different, can be acted upon by all influences, external and internal, in such diverse ways and proportions that the results (unequals being added to unequals) shall be equal and similar. Still, though highly improbable, this cannot be said to be impossible; and if there is an innate law of any kind helping to determine specific evolution, this may more or less, or entirely, neutralize or even reverse the effect of ancestral habit. Thus, it is quite conceivable that a pleurodont lizard might have arisen in Madagascar in perfect independence of the similarly-formed American lacertilia: just as certain teeth of carnivorous and insectivorous marsupial animals have been seen most closely to resemble those of carnivorous and insectivorous placental beasts; just as, again, the paddles of the Cetacea resemble, in the fact of a multiplication in the number of the phalanges, the many-jointed feet of extinct marine reptiles, and as the beak of the cuttle-fish or of the tadpole resembles that of birds. We have already seen (in Chapter III.) that it is impossible, upon any hypothesis, to escape admitting the independent origins of closely similar forms, It may be that they are both more frequent and more important than is generally thought.
That closely similar structures may arise without a genetic relationship has been lately well urged by Mr. Ray Lankester.[159] He has brought this notion forward even as regards the bones of the skull in osseous fishes and in mammals. He has done so on the ground that the probable common ancestor of mammals and of osseous fishes was a vertebrate animal of so low a type that it could not be supposed to have possessed a skull differentiated into distinct bony elements—even if it was bony at all. If this was so, then the cranial bones must have had an independent origin in each class, and in this case we have the most strikingly harmonious and parallel results from independent actions. For the bones of the skull in an osseous fish are so closely conformed to those of a mammal, that "both types of skull exhibit many bones in common," though "in each type some of these bones acquire special arrangements and very different magnitudes."[160] And no investigator of homologies doubts that a considerable number of the bones which form the skull of any osseous fish are distinctly homologous with the cranial bones of man. The occipital, the parietal, and frontal, the bones which surround the internal ear, the vomer, the premaxilla, and the quadrate bones, may be given as examples. Now, if such close relations of homology can be brought about independently of any but the most remote genetic affinity, it would be rash to affirm dogmatically that there is any impossibility in the independent origin of such forms as centetes and solenodon, or of genetically distinct batrachians, as similar to each other as are some of the frogs of South America and of Europe. At the same time such phenomena must at present be considered as very improbable, from the action of ancestral habit, as before stated.
We have seen, then, that the geographical distribution of animals presents difficulties, though not insuperable ones, for the Darwinian hypothesis. If, however, other reasons against it appear of any weight—if, especially, there is reason to believe that geological time has not been sufficient for it, then it will be well to bear in mind the facts here enumerated. These facts, however, are not opposed to the doctrine of evolution; and if it could be established that closely similar forms had really arisen in complete independence one of the other, they would rather tend to strengthen and to support that theory.
CHAPTER VIII.
HOMOLOGIES.
Animals made-up of parts mutually related in various ways.—What homology is.—Its various kinds.—Serial homology.—Lateral homology.—Vertical homology.—Mr. Herbert Spencer's explanations.—An internal power necessary, as shown by facts of comparative anatomy.—Of teratology.—M. St. Hilaire.—Professor Burt Wilder.—Foot-wings.—Facts of pathology.—Mr. James Paget.—Dr. William Budd.—The existence of such an internal power of individual development diminishes the improbability of an analogous law of specific origination.
That concrete whole which is spoken of as "an individual" (such, e.g., as a bird or a lobster) is formed of a more or less complex aggregation of parts which are actually (from whatever cause or causes) grouped, together in a harmonious interdependency, and which have a multitude of complex relations amongst themselves.
The mind detects a certain number of these relations as it contemplates the various component parts of an individual in one or other direction—as it follows up different lines of thought. These perceived relations, though subjective, as relations, have nevertheless an objective foundation as real parts, or conditions of parts, of real wholes; they are, therefore, true relations, such, e.g., as those between the right and left hand, between the hand and the foot, &c.
The component parts of each concrete whole have also a relation of resemblance to the parts of other concrete wholes, whether of the same or of different kinds, as the resemblance between the hands of two men, or that between the hand of a man and the fore-paw of a cat.
Now, it is here contended that the relationships borne one to another by various component parts, imply the existence of some innate, internal condition, conveniently spoken of as a power or tendency, which is quite as mysterious as is any innate condition, power, or tendency, resulting in the orderly evolution of successive specific manifestations. These relationships, as also this developmental power, will doubtless, in a certain sense, be somewhat further explained as science advances. But the result will be merely a shifting of the inexplicability a point backwards, by the intercalation of another step between the action of the internal condition or power and its external result. In the meantime, even if by "Natural Selection" we could eliminate the puzzles of the "origin of species," yet other phenomena, not less remarkable (namely, those noticed in this chapter), would still remain unexplained and as yet inexplicable. It is not improbable that, could we arrive at the causes conditioning all the complex inter-relations between the several parts of one animal, we should at the same time obtain the key to unlock the secrets of specific origination.
It is desirable, then, to see what facts there are in animal organization which point to innate conditions (powers and tendencies), as yet unexplained, and upon which the theory of "Natural Selection" is unable to throw any explanatory light.
The facts to be considered are the phenomena of "homology," and especially of serial, bilateral, and vertical homology.
The word "homology" indicates such a relation between two parts that they may be said in some sense to be "the same," or at least "of similar nature." This similarity, however, does not relate to the use to which parts are put, but only to their relative position with regard to other parts, or to their mode of origin. There are many kinds of homology,[161] but it is only necessary to consider the three kinds above enumerated.