In cases where a very old form of life has continued to exist unmodified, so that by investigation of its anatomy we are brought back to a more primitive type of structure than that of the newer forms growing higher up _upon the same branch_, two things are observable. In the first place, the old form is less differentiated than the newer ones; and, in the next place, it is seen much more closely to resemble types of structure belonging to some of the other and larger branches of the tree. The organization of the older form is not only _simpler_; but it is, as naturalists say, more _generalized_. It comprises within itself characters belonging to its own branch, and also characters belonging to neighbouring branches, or to the trunk from which allied branches spring. Hence it becomes a general rule of classification, that it is by the lowest, or by the oldest, forms of any two natural groups that the affinities between the two groups admit of being best detected. And it is obvious that this is just what ought to be the case on the theory of descent with divergent modification; while, upon the alternative theory of special creation, no reason can be assigned why the lowest or the oldest types should thus combine the characters which afterwards become severally distinctive of higher or newer types. Again, I have already alluded to the remarkable fact that there is no correlation between the value of structures to the organisms which present them, and their value to the naturalist for the purpose of tracing natural affinity; and I have remarked that up to the close of the last century it was regarded as an axiom of taxonomic science, that structures which are of most importance to the animals or plants possessing them must likewise prove of most importance in any natural system of classification. On this account, all attempts to discover the natural classification went upon the supposition that such a direct proportion must obtain--with the result that organs of most physiological importance were chosen as the bases of systematic work. And when, in the earlier part of the present century, De Candolle found that instead of a direct there was usually an inverse proportion between the functional and the taxonomic value of a structure, he was unable to suggest any reason for this apparently paradoxical fact. For, upon the theory of special creation, no reason can be assigned why organs of least importance to organisms should prove of most importance as marks of natural affinity. But on the theory of descent with progressive modification the apparent paradox is at once explained. For it is evident that organs of functional importance are, other things equal, the organs which are most likely to undergo different modifications in different lines of family descent, and therefore in time to have their genetic relationships in these different lines obscured. On the other hand, organs or structures which are of no functional importance are