Selection" only it is so improbable as to be practically impossible for two exactly-similar structures to have ever been independently developed. It is so because the number of possible variations is indefinitely great, and it is therefore an indefinitely great number to one against a similar series of variations occurring and being similarly preserved in any two independent instances.

The difficulty here asserted applies, however, only to pure Darwinism, which makes use only of indirect modifications through the survival of the fittest.

Other theories (for example, that of Mr. Herbert Spencer) admit the direct action of conditions upon animals and plants—in ways not yet fully understood there being conceived to be at the same time a certain peculiar but limited power of response and adaptation in each animal and plant so acted on. Such theories have not to contend against the difficulty proposed, and it is here urged that even very complex extremely similar structures have again and again been developed quite independently one of the other, and this because the process has taken place not by merely haphazard, indefinite variations in all directions, but by the concurrence of some other and internal natural law or laws coöperating with external influences and with "Natural Selection" in the evolution of organic forms.

It must never be forgotten that to admit any such constant operation of any such unknown natural cause is to deny the purely Darwinian theory, which relies upon the survival of the fittest by means of minute fortuitous indefinite variations.

Among many other obligations which the author has to acknowledge to Prof. Huxley are, the pointing out of this very difficulty, and the calling his attention to the striking resemblance between certain teeth of the dog and of the thylacine as one instance, and certain ornithic peculiarities of pterodactyls as another.

Mammals 3 are divisible into one great group, which comprises the immense majority of kinds termed, from their mode of reproduction, placental Mammals, and into another very much smaller group comprising the pouchedbeasts or marsupials (which are the kangaroos, bandicoots, phalangers, etc., of Australia), and the true opossums of America, called implacental Mammals. Now, the placental mammals are subdivided into various orders, among which are the flesh-eaters (Carnivora, i. e., cats, dogs, otters, weasels, etc.), and the insect-eaters (Insectivora, i. e., moles, hedgehogs, shrew-mice, etc.). The marsupial mammals also present a variety of forms (some of which are carnivorous beasts, while others are insectivorous), so marked that it has been even proposed to divide them into orders parallel to the orders of placental beasts.

The resemblance, indeed, is so striking as, on Darwinian principles, to suggest the probability of genetic affinity; and it even led Prof. Huxley, in his Hunterian Lectures, in 1866, to promulgate the notion that a vast and widely-diffused marsupial fauna may have existed anteriorly to the

W

TEETH OF UROTRICHUS AND PERAMELES

development of the ordinary placental, non-pouched beasts, and that the carnivorous, insectivorous, and herbivorous.

3 I. e., warm-blooded animals which suckle their young, such as apes, bats, hoofed beasts, lions, dogs, bears, weasels, rats, squirrels, armadillos, sloths, whales, porpoises, kangaroos, opossums, etc.

placentals may have respectively descended from the carnivorous, insectivorous, and herbivorous marsupials.

Among other points Prof. Huxley called attention to the resemblance between the anterior molars of the placental dog with those of the marsupial thylacine. These, indeed, are strikingly similar, but there are better examples still of this sort of coincidence. Thus it has often been remarked that the insectivorous marsupials, e. g., Perameles, wonderfully correspond, as to the form of certain of the grinding teeth, with certain insectivorous placentals, e. g., Urotrichus.

Again, the saltatory insectivores of Africa (Macroscelides) not only resemble the kangaroo family (Macropodida) in their jumping habits and long hind-legs, but also in the structure of their molar teeth, and even further, as I have elsewhere* pointed out, in a certain similarity of the upper cutting teeth, or incisors.

Now, these correspondences are the more striking when we bear in mind that a similar dentition is often put to very different uses. The food of different kinds of apes is very different, yet how uniform is their dental structure! Again, who, looking at the teeth of different kinds of bears, would ever suspect that one kind was frugivorous, and another a devourer exclusively of animal food?

The suggestion made by Prof. Huxley was therefore one which had much to recommend it to Darwinians, though it has not met with any notable acceptance, and though he seems himself to have returned to the older notion, namely, that the pouched-beasts, or marsupials, are a special ancient offshoot from the great mammalian class.

But, whichever view may be the correct one, we have in either case a number of forms similarly modified in harmony with surrounding conditions, and eloquently proclaiming some natural plastic power, other than mere fortuitous

4 " Journal of Anatomy and Physiology " (1868), vol. ii., p. 139.

variation with survival of the fittest. If, however, the reader thinks that teeth are parts peculiarly qualified for rapid variation (in which view the author cannot concur), he is requested to suspend his judgment till he has considered the question of the independent evolution of the highest organs of sense. If this seems to establish the existence of some other law than that of "Natural Selection," then the operation of that other law may surely be also traced in the harmonious coördinations of dental form.

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 class of birds is one which is remarkably uniform in its organization. So much is this the case, that the best mode of subdividing the class 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, cassowary, apteryx, .dinornis, etc. 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 or Darwinian principles it is impossible that the class of birds so uniform and homogeneous 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 pterodactyls, or forms allied to them, were the progenitors of the class of birds; and certain parts of their structure especially support this view. Allusion is here made to the blade-bone (scapula) and the bone which passes down from the shoulder-joint to the breast-bone (viz., the coracoid). These bones are such remarkable anticipations of the same parts in ordinary (i. e., carinate) birds 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 Prof. 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 has shown that in pterodac tyls, 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, as also by Prof. Cope in the United States, that the line of descent from reptiles to birds has not been from ordinary reptiles, through pterodactyl-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 Pterodactyls with carinate birds; (2) that

6

5

5 See "Ann. and Mag. of Nat. Hist." for August, 1870, p. 140.

6 See "Proceedings of the Royal Institution," vol. v., part iv., p. 278: Report of a Lecture delivered February 7, 1868. Also "Quarterly Journal of the Geological Society," February, 1870. "Contributions to the Anatomy and Taxonomy of the Dinosauria."