Let us next turn to another of Darwin's general rules with reference to distribution. He took a great deal of trouble to collect evidence of the two following facts, namely, (1) that "species of the larger genera in each country vary more frequently than the species of the smaller genera"; and (2) that "many of the species included within the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges[22]." By larger genera he means genera containing many species; and he accounts for these general facts by the principle, "that where many species of a genus have been formed, on an average many are still forming." But _how_ forming? If we say by natural selection alone, we should expect to find the multitudinous species differing from one another in respect of features presenting well-marked adaptive meanings; yet this is precisely what we do not find. For Darwin's argument here is that "in large genera the amount of difference between the species is often exceedingly small, so that in this respect the species of the larger genera resemble varieties more than do the species of the smaller genera." Therefore the argument, while undoubtedly a very forcible one in favour of the fact of _evolution_, appears to me scarcely consistent with the view of this evolution being due solely to natural selection. On the other hand, the argument tells strongly (though unconsciously) in favour of physiological selection. For the larger a genus, or the greater the number of its species, the greater must be the opportunity for the occurrence of that particular kind of variation on which the principle of physiological selection depends. The species of a genus may be regarded as so many varieties which have already been separated from one another physiologically; therefore each of them may now constitute a new starting-point for a further and similar separation--particularly as, in virtue of their previous segregation, many are now exposed to different conditions of life. Thus, it seems to me, we can well understand why it is that genera already rich in species tend to grow richer; while such is not the case in so great a degree with genera that are poor in species. Moreover, we can well understand that, multiplication of species being as a rule, and in the first instance, determined by changes in the reproductive system, wherever a large number of new species are being turned out, the secondary differences between them should be "often exceedingly small"--a general correlation which, so far as I can see, we are not able to understand on the theory of natural selection. [22] _Origin of Species_, pp. 44, 45.