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 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 Institution 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."
It is quite consistent, then, in the Professor to explain the
difficulty as he does; but it would not be similarly so with an absolute and pure Darwinian.
Yet stronger arguments of an analogous 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 pass 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 pass from the cerebral ganglia into the cartilaginous chambers to reach the auditory sacs. Moreover,
it has been suggested by Professor Owen that sinuosities between processes projecting from the inner wall of each chamber "seem to be the first rudiments of those which, in the higher classes (i.e. in animals with a spinal column), are extended in the form of canals and spiral chambers, within the substance of the dense nidus of the labyrinth."[[59]]