The criteria of these primordial forms or species of nature, are, (1.) Morphological. Animals, however, may approach very nearly in their structure, and yet belong to different species. It is only when the peculiarities of structure are indicative of specialty of design, that they form a safe ground of classification. If the teeth of one animal are formed to fit it to feed on flesh, and those of another to fit it to feed on plants; if one has webbed feet and another not; then, in all such cases, difference of structure proves difference of kind. (2.) Physiological; that is, the internal nature, indicated by habits and instincts, furnishes another safe criterion. (3.) Permanent fecundity. The progenitors of the same species reproduce their kind from generation to generation; the progeny of different species, although nearly allied, do not. It is a fixed law of nature that species never can be annihilated, except by all the individuals included in them dying out; and that new species cannot be produced. Every true species is primordial. It is this fact, that is, that no variety, with the essential characteristics of species, has ever been produced, that forces, as we saw above, Professor Huxley to pronounce Mr. Darwin's doctrine to be an unproved hypothesis. Species continue; varieties, if let alone, always revert to the normal type. It requires the skill and constant attention of man to keep them distinct. Now that there are such forms in nature, is proved not only from the testimony of the great body of the most distinguished naturalists, but by all the facts in the case. First, the fact that such species are known to have existed unchanged, through what geologists consider almost immeasurable periods of time. Palaeontologists tell us that Trilobites abounded from the primordial age down to the Carboniferous period, that is, as they suppose, through millions of years. More wonderful still, the little animals whose remains constitute the chalk formations which are spread over large areas of country, and are sometimes a hundred feet thick, are now at work at the bottom of the Atlantic. Principal Dawson tells us, with regard to Mollusks existing in a sub-fossil state in the Post-pliocene clays of Canada, that "after carefully studying about two hundred species, and of some of these, many hundreds of specimens, I have arrived at the conclusion that they are absolutely unchanged.... Here again we have an absolute refusal, on the part of all these animals, to admit that they are derived, or have tended to sport into new species."[51] On the previous page he says, "Pictet catalogues ninety-eight species of mammals which inhabited Europe in the Post-glacial period. Of these fifty-seven still exist unchanged, and the remainder have disappeared. Not one can be shown to have been modified into a new form, though some of them have been obliged, by changes of temperature and other conditions, to remove into distant and now widely separated regions."