that these characters have been inherited from a common ancestor; and we know that such aggregated characters have especial value in classification[2]." [2] _Origin of Species_, p. 372. It is true that even a single character, if found common to a large number of forms, while uniformly absent from others, is also regarded by naturalists as of importance for purposes of classification, although they recognise it as of a value subordinate to that of aggregates of characters. But this also is what we should expect on the theory of descent. If even any one structure be found to run through a number of animals presenting different habits of life, the readiest explanation of the fact is to be found in the theory of descent; but this does not hinder that if several such characters always occur together, the inference of genetic relationship is correspondingly confirmed. And the fact that before this inference was ever drawn, naturalists recognised the value of single characters in proportion to their constancy, and the yet higher value of aggregates of characters in proportion to their number--this fact shows that in their work of classification naturalists empirically observed the effects of a cause which we have now discovered, to wit, hereditary transmission of characters through ever-widening groups of changing species. There is another argument which appears to tell strongly in favour of the theory of descent. We have just seen that non-adaptive structures, not being required to change in response to change of habits or conditions of life, are allowed to persist unchanged through many generations, and thus furnish exceptionally good guides in the science of classification--or, according to our theory, in the work of tracing lines of pedigree. But now, the converse of this statement holds equally true. For it often happens that adaptive structures are required to change in different lines of descent in analogous ways, in order to meet analogous needs; and, when such is the case, the structures concerned have to assume more or less close resemblances to one another, even though they have severally descended from quite different ancestors. The paddles of a whale, for instance, most strikingly resemble the fins of a fish as to their outward form and movements; yet, on the theory of descent, they must be held to have had a widely different parentage. Now, in all such cases where there is thus what is called an analogous (or adaptive) resemblance, as distinguished from what is called an homologous (or anatomical) resemblance--in all such cases it is observable that the similarities do not extend further into the structure of the parts than it is necessary that they should extend, in order that the structures should both perform the same functions. The whole anatomy of the paddles of a whale is quite unlike that of the fins of a fish--being, in fact, that of the fore-limb of a mammal. The