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.
Amongst many other obligations which the Author has to acknowledge to Professor 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 pterodactyles as another.
Mammals[[51]] 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 pouched-beasts or marsupials (which are the kangaroos, bandicoots, phalangers, &c., of Australia), and the true opossums of America, called implacental Mammals. Now the placental mammals are subdivided into various orders, amongst which are the flesh-eaters (Carnivora, i.e. cats, dogs, otters, weasels, &c.), and the insect-eaters (Insectivora, i.e. moles, hedgehogs, shrew-mice, &c.). The marsupial mammals also present a variety of forms (some of which are carnivorous beasts, whilst 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 Professor 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 development of the ordinary placental, non-pouched beasts, and that the carnivorous, insectivorous, and herbivorous placentals may have respectively descended from the carnivorous, insectivorous, and herbivorous marsupials.
Amongst other points Professor 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 (Macropodidæ) in their jumping habits and long hind legs, but also in the structure of their molar teeth, and even further, as I have elsewhere[[52]] 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?