The other difficulty, kindly suggested to me by the learned Professor, refers to the structure of birds, and of extinct reptiles more or less related to them.
The cla.s.s of birds is one which is remarkably uniform in its organization.
So much is this the case, that the best mode of subdividing the cla.s.s is a problem of the greatest difficulty. Existing birds, however, present forms which, though closely resembling in the greater part of their structure, yet differ importantly the one from the other. One form is exemplified by the ostrich, rhea, emeu, ca.s.sowary, apteryx, dinornis, &c. These are the _struthious_ birds. All other existing birds belong to the second division, and are called (from the keel on the breast-bone) _carinate_ birds.
Now birds and reptiles have such and so many points in common, that Darwinians must regard the former as modified descendants of ancient reptilian forms. But on Darwinian principles it is impossible that the cla.s.s of birds so uniform and h.o.m.ogeneous should have had a double reptilian origin. If one set of birds sprang from one set of reptiles, and another set of birds from another set of reptiles, the two sets could never, by "Natural Selection" only, have grown into such a perfect similarity. To admit such a phenomenon would be equivalent to abandoning the theory of "Natural Selection" as the sole origin of species.
Now, until recently it has generally been supposed by evolutionists that those ancient flying reptiles, the pterodactyles, or forms allied to them, were the progenitors of the cla.s.s of birds; and certain parts of their structure especially support this view. Allusion is here made to the bladebone (scapula), and the bone which pa.s.ses down from the shoulder-joint to the breast-bone (viz. the coracoid). These bones are such remarkable antic.i.p.ations of the same parts in ordinary (_i.e._ carinate) birds {71} that it is hardly possible for a Darwinian not to regard the resemblance as due to community of origin. This resemblance was carefully pointed out by Professor Huxley in his "Hunterian Course" for 1867, when attention was called to the existence in _Dimorphodon macronyx_ of even that small process which in birds gives attachment to the upper end of the merrythought. Also Mr. Seeley[53] has shown that in pterodactyles, as in birds, the optic lobes of the brain were placed low down on each side--"lateral and depressed." Nevertheless, the view has been put forward and ably maintained by the same Professor,[54] as also by Professor Cope in the United States, that the line of descent from reptiles to birds has not been from ordinary reptiles, through pterodactyle-like forms, to ordinary birds, but to the struthious ones from certain extinct reptiles termed Dinosauria; one of the most familiarly known of which is the Iguanodon of the Wealden formation. In these Dinosauria we find skeletal characters unlike those of ordinary (_i.e._ carinate) birds, but closely resembling in certain points the osseous structure of the struthious birds. Thus a difficulty presents itself as to the explanation of the three following relationships:--(1) That of the Pterodactyles with carinate birds; (2) that of the Dinosauria with struthious birds; (3) that of the carinate and struthious birds with each other.
Either birds must have had two distinct origins whence they grew to their present conformity, or the very same skeletal, and probably cerebral characters must have spontaneously and independently arisen. Here is a dilemma, either horn of which bears a threatening aspect to the exclusive supporter of "Natural Selection," and between which it seems somewhat {72} difficult to choose.
It has been suggested to me that this difficulty may be evaded by considering pterodactyles and carinate birds as independent branches from one side of an ancient common trunk, while similarly the Dinosauria and struthious birds are taken to be independent branches from the other side of the same common trunk; the two kinds of birds resembling each other so much on account of their later development from that trunk as compared with the development of the reptilian forms. But to this it may be replied that the ancient common stock could not have had at one and the same time a shoulder structure of _both kinds_. It must have been that of the struthious birds or that of the carinate birds, or something different from both. If it was that of the struthious birds, how did the pterodactyles and carinate birds independently arrive at the very same divergent structure?
If it was that of the carinate birds, how did the struthious birds and Dinosauria independently agree to differ? Finally, if it was something different from either, how did the carinate birds and pterodactyles take on independently one special common structure when disagreeing in so many; while the struthious birds, agreeing in many points with the Dinosauria, agree yet more with the carinate birds? Indeed by no arrangement of branches from a stem can the difficulty be evaded.
Professor Huxley seems inclined[55] to cut the Gordian knot by considering the shoulder structure of the pterodactyle as independently educed, and having relation to physiology only. This conception is one which harmonizes completely with the views here advocated, and with those of Mr. Herbert Spencer, who also calls in direct modification to the aid of "Natural Selection." That merely minute, indefinite variations in all directions should unaided have independently built up the shoulder structure of {73} the pterodactyles and carinate birds, and have laterally depressed their optic lobes, at a time so far back as the deposition of the Oolite strata,[56] is a coincidence of the highest improbability; but that an innate power and evolutionary law, aided by the corrective action of "Natural Selection," should have furnished like needs with like aids, is not at all improbable. The difficulty does not tell against the theory of evolution, but only against the specially Darwinian form of it. Now this form has never been expressly adopted by Professor Huxley; so far from it, in his lecture on this subject at the Royal Inst.i.tution before referred to, he observes,[57] "I can testify, from personal experience, it is possible to have a complete faith in the general doctrine of evolution, and yet to hesitate in accepting the Nebular, or the Uniformitarian, or the Darwinian hypotheses in all their integrity and fulness."
[Ill.u.s.tration: THE ARCHEOPTERYX (of the Oolite strata).]
It is quite consistent, then, in the Professor to explain the {74} difficulty as he does; but it would not be similarly so with an absolute and pure Darwinian.
Yet stronger arguments of an a.n.a.logous kind are, however, to be derived from the highest organs of sense. In the most perfectly organized animals--those namely which, like ourselves, possess a spinal column--the internal organs of hearing consist of two more or less complex membranous sacs (containing calcareous particles--otoliths), which are primitively or permanently lodged in two chambers, one on each side of the cartilaginous skull. The primitive cartilaginous cranium supports and protects the base of the brain, and the auditory nerves pa.s.s from that brain into the cartilaginous chambers to reach the auditory sacs. These complex arrangements of parts could not have been evolved by "Natural Selection,"
_i.e._ by minute accidental variations, except by the action of such through a vast period of time; nevertheless, it was fully evolved at the time of the deposition of the upper Silurian rocks.
Cuttle-fishes (_Cephalopoda_) are animals belonging to the molluscous primary division of the animal kingdom, which division contains animals formed upon a type of structure utterly remote from that on which the animals of the higher division provided with a spinal column are constructed. And indeed no transitional form (tending even to bridge over the chasm between these two groups) has ever yet been discovered, either living or in a fossilized condition.[58]
Nevertheless, in the two-gilled Cephalopods (_Dibranchiata_) we find the brain supported and protected by a cartilaginous cranium. In the base of this cranium are two cartilaginous chambers. In each chamber is a membranous sac containing an otolith, and the auditory nerves pa.s.s from the cerebral ganglia into the cartilaginous chambers to reach the auditory sacs. Moreover, it has been suggested by Professor Owen that {75} sinuosities between processes projecting from the inner wall of each chamber "seem to be the first rudiments of those which, in the higher cla.s.ses (_i.e._ in animals with a spinal column), are extended in the form of ca.n.a.ls and spiral chambers, within the substance of the dense nidus of the labyrinth."[59]
[Ill.u.s.tration: CUTTLE-FISH.
A. Ventral aspect. B. Dorsal aspect.]
Here, then, we have a wonderful coincidence indeed; two highly complex auditory organs, marvellously similar in structure, but which must nevertheless have been developed in entire and complete independence one of the other! It would be difficult to calculate the odds against the independent occurrence and conservation of two such complex series of merely accidental and minute haphazard variations. And it can never be {76} maintained that the sense of hearing could not be efficiently subserved otherwise than by such sacs, in cranial cartilaginous capsules so situated in relation to the brain, &c.
Our wonder, moreover, may be increased when we recollect that the two-gilled cephalopods have not yet been found below the lias, where they at once abound; whereas the four-gilled cephalopods are Silurian forms.
Moreover, the absence is in this case significant in spite of the imperfection of the geological record, because when we consider how many individuals of various kinds of four-gilled cephalopods have been found, it is fair to infer that at the least a certain small percentage of dibranchs would also have left traces of their presence had they existed. Thus it is probable that some four-gilled form was the progenitor of the dibranch cephalopods. Now the four-gilled kinds (judging from the only existing form, the nautilus) had the auditory organ in a very inferior condition of development to what we find in the dibranch; thus we have not only evidence of the independent high development of the organ in the former, but also evidence pointing towards a certain degree of comparative rapidity in its development.
Such being the case with regard to the organ of hearing, we have another yet stronger argument with regard to the organ of sight, as has been well pointed out by Mr. J. J. Murphy.[60] He calls attention to the fact that the eye must have been perfected in at least "three distinct lines of descent," alluding not only to the molluscous division of the animal kingdom, and the division provided with a spinal column, but also to a third primary division, namely, that which includes all insects, spiders, crabs, &c., which are spoken of as Annulosa, and the type of whose structure is as distinct from that of the molluscous type on the one hand, as it is from that of the type with a spinal column (_i.e._ the vertebrate type) on the other.
{77} In the cuttle-fishes we find an eye even more completely constructed on the vertebrate type than is the ear. Sclerotic, retina, choroid, vitreous humour, lens, aqueous humour, all are present. The correspondence is wonderfully complete, and there can hardly be any hesitation in saying that for such an exact, prolonged, and correlated series of similar structures to have been brought about in two independent instances by merely indefinite and minute accidental variations, is an improbability which amounts practically to impossibility. Moreover, we have here again the same imperfection of the four-gilled cephalopod, as compared with the two-gilled, and therefore (if the latter proceeded from the former) a similar indication of a certain comparative rapidity of development.
Finally, and this is perhaps one of the most curious circ.u.mstances, the process of formation appears to have been, at least in some respects, the same in the eyes of these molluscous animals as in the eyes of vertebrates.
For in these latter the cornea is at first perforated, while different degrees of perforation of the same part are presented by different adult cuttle-fishes--large in the calamaries, smaller in the octopods, and reduced to a minute foramen in the true cuttle-fish sepia.
Some may be disposed to object that the conditions requisite for effecting vision are so rigid that similar results in all cases must be independently arrived at. But to this objection it may well be replied that Nature herself has demonstrated that there is no such necessity as to the details of the process. For in the higher Annulosa, such as the dragon-fly, we meet with an eye of an unquestionably very high degree of efficiency, but formed on a type of structure only remotely comparable with that of the fish or the cephalopod. The last-named animal might have had an eye as efficient as that of a vertebrate, but formed on a distinct type, instead of being another edition, as it were, of the very same structure.
In the beginning of this chapter examples have been given of the very {78} diverse mode in which similar results have in many instances been arrived at; on the other hand, we have in the fish and the cephalopod not only the eye, but at one and the same time the ear also similarly evolved, yet with complete independence.
Thus it is here contended that the similar and complex structures of both the highest organs of sense, as developed in the vertebrates on the one hand, and in the mollusks on the other, present us with residuary phenomena for which "Natural Selection" alone is quite incompetent to account. And that these same phenomena must therefore be considered as conclusive evidence for the action of some other natural law or laws conditioning the simultaneous and independent evolution of these harmonious and concordant adaptations.
Provided with this evidence, it may be now profitable to enumerate other correspondences, which are not perhaps in themselves inexplicable by Natural Selection, but which are more readily to be explained by the action of the unknown law or laws referred to--which action, as its necessity has been demonstrated in one case, becomes _a priori_ probable in the others.
[Ill.u.s.tration: SKELETON OF AN ICHTHYOSAURUS.]
Thus the great oceanic Mammalia--the whales--show striking resemblances to those prodigious, extinct, marine reptiles, the Ichthyosauria, and this not only in structures readily referable to similarity of habit, but in such matters as greatly elongated premaxillary bones, together with the concealment of certain bones of the skull by other cranial bones. [Page 79]
Again, the aerial mammals, the bats, resemble those flying reptiles of the secondary epoch, the pterodactyles; not only to a certain extent in the breast-bone and mode of supporting the flying membrane, but also in the proportions of different parts of the spinal column and the hinder (pelvic) limbs.
Also bivalve sh.e.l.l-fish (_i.e._ creatures of the mussel, c.o.c.kle, and oyster cla.s.s, which receive their name from the body being protected by a double sh.e.l.l, one valve of which is placed on each side) have their two sh.e.l.ls united by one or two powerful muscles, which pa.s.s directly across from one sh.e.l.l to the other, and which are termed "adductor muscles" because by their contraction they bring together the valves and so close the sh.e.l.l.
[Ill.u.s.tration: CYTHERIDEA TOROSA.
[An ostracod (Crustacean), externally like a bivalve sh.e.l.l-fish (Lamellibranch).]
Now there are certain animals which belong to the crab and lobster cla.s.s (Crustacea)--a cla.s.s constructed on an utterly different type from that on which the bivalve sh.e.l.l-fish are constructed--which present a very curious approximation to both the form and, in a certain respect, the structure of true bivalves. Allusion is here made to certain small Crustacea--certain phyllopods and ostracods--which have the hard outer coat of their thorax so modified as to look wonderfully like a bivalve sh.e.l.l, although its {80} nature and composition are quite different. But this is by no means all,--not only is there this external resemblance between the thoracic armour of the crustacean and the bivalve sh.e.l.l, but the two sides of the ostracod and phyllopod thorax are connected together also by an adductor muscle!
[Ill.u.s.tration: A POLYZOON WITH BIRD"S-HEAD PROCESSES.]
{81} The pedicellariae of the echinus have been already spoken of, and the difficulty as to their origin from minute, fortuitous, indefinite variations has been stated. But structures essentially similar (called avicularia, or "bird"s-head processes") are developed from the surface of the compound ma.s.ses of certain of the highest of the polyp-like animals (viz. the Polyzoa or, as they are sometimes called, the Bryozoa).
These compound animals have scattered over the surface of their bodies minute processes, each of which is like the head of a bird, with an upper and lower beak, the whole supported on a slender neck. The beak opens and shuts at intervals, like the jaws of the pedicellariae of the echinus, and there is altogether, in general principle, a remarkable similarity between the structures. Yet the echinus can have, at the best, none but the most distant genetic relationship with the Polyzoa. We have here again therefore complex and similar organs of diverse and independent origin.
[Ill.u.s.tration: BIRD"S-HEAD PROCESSES VERY GREATLY ENLARGED.]
In the highest cla.s.s of animals (the Mammalia) we have almost always a placental mode of reproduction, _i.e._ the blood of the foetus is placed in nutritive relation with the blood of the mother by means of vascular prominences. No trace of such a structure exists in any bird or in any reptile, and yet it crops out again in certain sharks. There indeed it might well be supposed to end, but, marvellous as it seems, it reappears in very lowly creatures; namely, in certain of the ascidians, sometimes called tunicaries or sea-squirts. [Page 82]
Now, if we were to concede that the ascidians were the common ancestors[61]
of both these sharks and of the higher mammals, we should be little, if any, nearer to an explanation of the phenomenon by means of "Natural Selection," for in the sharks in question the vascular prominences are developed from one foetal structure (the umbilical vesicle), while in the higher mammals they are developed from quite another part, viz. the allantois.
[Ill.u.s.tration: Upper Figure--ANTECHINUS MINUTISSIMUS (_implacental_).
Lower Figure--MUS DELICATULUS (_placental_).]
So great, however, is the number of similar, but apparently independent, structures, that we suffer from a perfect _embarras de richesses_. Thus, for example, we have the convoluted windpipe of the sloth, reminding us{83} of the condition of the windpipe in birds; and in another mammal, allied to the sloth, namely the great ant-eater (Myrmecophaga), we have again an ornithic character in its h.o.r.n.y gizzard-like stomach. In man and the highest apes the caec.u.m has a vermiform appendix, as it has also in the wombat!
Also the similar forms presented by the crowns of the teeth in some seals, in certain sharks, and in some extinct Cetacea may be referred to; as also the similarity of the beak in birds, some reptiles, in the tadpole, and cuttle-fishes. As to entire external form, may be adduced the wonderful similarity between a true mouse (_Mus delicatulus_) and a small marsupial, pointed out by Mr. Andrew Murray in his work on the "Geographical Distribution of Mammals," p. 53, and represented in the frontispiece by figures copied from Gould"s "Mammals of Australia;" but instances enough for the present purpose have been already quoted.
Additional reasons for believing that similarity of structure is produced by other causes than merely by "Natural Selection" are furnished by certain facts of zoological geography, and by a similarity in the mode of variation being sometimes extended to several species of a genus, or even to widely different groups; while the restriction and the limitation of such similarity are often not less remarkable. Thus Mr. Wallace says,[62] as to local influence: "Larger or smaller districts, or even single islands, give a special character to the majority of their Papilionidae. For instance:--1.
The species of the Indian region (Sumatra, Java, and Borneo) are almost invariably smaller than the allied species inhabiting Celebes and the Moluccas. 2. The species of New Guinea and Australia are also, though in a less degree, smaller than the nearest species or varieties of the Moluccas.
3. In the Moluccas themselves the species of Amboyna are the largest. 4.
The species of Celebes equal or even surpa.s.s in size those of Amboyna. {84} 5. The species and varieties of Celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands. 6. Tailed species in India or the Indian region become tailless as they spread eastward through the Archipelago. 7. In Amboyna and Ceram the females of several species are dull-coloured, while in the adjacent islands they are more brilliant."
Again:[63] "In Amboyna and Ceram the female of the large and handsome _Ornithoptera Helena_ has the large patch on the hind wings constantly of a pale dull ochre or buff colour; while in the scarcely distinguishable varieties from the adjacent islands, of Bouru and New Guinea, it is of a golden yellow, hardly inferior in brilliancy to its colour in the male s.e.x.
The female of _Ornithoptera Priamus_ (inhabiting Amboyna and Ceram exclusively) is of a pale dusky brown tint, while in all the allied species the same s.e.x is nearly black, with contracted white markings. As a third example, the female of _Papilio Ulysses_ has the blue colour obscured by dull and dusky tints, while in the closely allied species from the surrounding islands, the females are of almost as brilliant an azure blue as the males. A parallel case to this is the occurrence, in the small islands of Goram, Matabello, Ke, and Aru, of several distinct species of Euploea and Diadema, having broad bands or patches of white, which do not exist in any of the allied species from the larger islands. These facts seem to indicate some local influence in modifying colour, as unintelligible and almost as remarkable as that which has resulted in the modifications of form previously described."
After endeavouring to explain some of the facts in a way to be noticed directly, Mr. Wallace adds:[64] "But even the conjectural explanation now given fails us in the other cases of local modification. Why the species of the Western Islands should be smaller than those further east; why those of Amboyna should exceed in size those of Gilolo and New Guinea; why the {85} tailed species of India should begin to lose that appendage in the islands, and retain no trace of it on the borders of the Pacific; and why, in three separate cases, the females of Amboyna species should be less gaily attired than the corresponding females of the surrounding islands, are questions which we cannot at present attempt to answer. That they depend, however, on some general principle is certain, because a.n.a.logous facts have been observed in other parts of the world. Mr. Bates informs me that, in three distinct groups, Papilios, which, on the Upper Amazon, and in most other parts of South America, have spotless upper wings, obtain pale or white spots at Para and on the Lower Amazon, and also that the aeneas group of Papilios never have tails in the equatorial regions and the Amazon valley, but gradually acquire tails in many cases as they range towards the northern or southern tropic. Even in Europe we have somewhat similar facts, for the species and varieties of b.u.t.terflies peculiar to the Island of Sardinia are generally smaller and more deeply coloured than those of the mainland, and the same has been recently shown to be the case with the common tortoisesh.e.l.l b.u.t.terfly in the Isle of Man; while _Papilio Hospiton_, peculiar to the former island, has lost the tail, which is a prominent feature of the closely allied _P. Machaon_.
"Facts of a similar nature to those now brought forward would no doubt be found to occur in other groups of insects, were local faunas carefully studied in relation to those of the surrounding countries; and they seem to indicate that climate and other physical causes have, in some cases, a very powerful effect in modifying specific form and colour, and thus directly aid in producing the endless variety of nature."
[Ill.u.s.tration: OUTLINES OF WINGS OF b.u.t.tERFLIES OF CELEBES COMPARED WITH THOSE OF ALLIED SPECIES ELSEWHERE.
Outer outline, _Papilio gigon_, of Celebes. Inner outline, _P. demolion_, of Singapore and Java.--2. Outer outline, _P. miletus_, of Celebes. Inner outline, _P. sarpedon_, India.--3. Outer outline, _Tachyris zarinda_, Celebes. Inner outline, _T. nero_.]
With regard to b.u.t.terflies of Celebes belonging to different families, they present "a peculiarity of outline which distinguishes them at a glance from those of any other part of the world:"[65] it is that the upper wings {86} are generally more elongated and the anterior margin more curved. Moreover, there is, in most instances, near the base an abrupt bend or elbow, which in some species is very conspicuous. Mr. Wallace endeavours to explain {87} this phenomenon by the supposed presence at some time of special persecutors of the modified forms, supporting the opinion by the remark that small, obscure, very rapidly flying and mimicked kinds have not had the wing modified. Such an enemy occasioning increased powers of flight, or rapidity in turning, he adds, "one would naturally suppose to be an insectivorous bird; but it is a remarkable fact that most of the genera of fly-catchers of Borneo and Java on the one side, and of the Moluccas on the other, are almost entirely absent from Celebes. Their place seems to be supplied by the caterpillar-catchers, of which six or seven species are known from Celebes, and are very numerous in individuals. We have no positive evidence that these birds pursue b.u.t.terflies on the wing, but it is highly probable that they do so when other food is scarce. Mr. Bates suggested to me that the larger dragon-flies prey upon b.u.t.terflies, but I did not notice that they were more abundant in Celebes than elsewhere."[66]