In particular, we have seen that the most archaic types are both simpler in their organization and more generalized in their characters than are the more recent types--a fact of which no explanation can be given on the theory of special creation. But, upon the theory of natural evolution, we can without difficulty understand why the earlier forms should have been the simpler forms, and also why they should have been the most generalized. For it is out of the older forms that the newer must have grown; and, as they multiplied, they must have become more and more differentiated. Again, we have seen that there is no correlation between the importance of any structure from a classificatory point of view, and the importance of that structure to the organism which presents it. On the contrary, it is a general rule that "the less any part of the organization is concerned with special habits, the more important it becomes for classification." Now, from the point of view of special creation it is unintelligible why unity of ideal should be most manifested by least important structures, whereas from the point of view of evolution it is to be expected that these life-serving structures should have been most liable to divergent modification in divergent lines of descent, or in adaptation to different conditions of life, while the trivial or less important characters should have been allowed to remain unmodified. Thus we can now understand why all primitive classifications were wrong in principle when they went upon the assumption that divine ideals were best exhibited by resemblances between life-serving (and therefore adaptive) structures, with the result that whales were classed with fishes, birds with bats, and so on. Nevertheless, these primitive naturalists were quite logical; for, from the premises furnished by the theory of special creation, it is much more reasonable to expect that unity of ideal should be shown in plainly adaptive characters than in trivial and more or less hidden anatomical characters. Moreover, long after biological science had ceased consciously to follow any theological theory, the apparent axiom continued to be entertained, that structures of most importance to organisms must also be structures of most importance to systematists. And when at last, in the present century, this was found not to be the case, no reason could be suggested why it was not the case. But now we are able fully to explain this apparent anomaly.