for possession of the _female_, and the _means of procreation_. The one is connected with the nutritive appetite, the other with the reproductive appetite. This mode of selection acts in two ways, by the law of battle and the law of attractiveness. The strongest or the most attractive males alone, or mainly, leave offspring, which, of course, inherit their peculiarities; and these are increased indefinitely by integration through successive generations, thus increasing the strength or the beauty. Of these two laws, the law of battle is most conspicuous among mammals, and the law of attractiveness among birds. It is evident that this factor can not operate among many lower animals which are hermaphroditic, nor among plants. Of these acknowledged factors of evolution, the first two were known to Lamarck and the older evolutionists. The third and fourth are distinctively Darwinian. According to Darwin, while all these are operative, the third is the most powerful; but Spencer accords this distinction to the Lamarckian factors. Many American zoölogists take the same view. Such until very recently were all the recognized factors of evolution. But, within the past year (1886) has taken place, it seems to us, the most important advance in the theory of evolution since Darwin. It is the suggestion by Mr. Catchpool,[14] and afterward the more full elaboration by Dr. Romanes, of another factor, which he calls “_physiological selection_.”[15] The great objections to the sufficiency of the theory of evolution, as left by Darwin, were twofold: 1. While natural selection accounts completely for the formation of _useful_ structures or adaptive modifications, and therefore for differences characterizing classes, orders, families, and even genera--for these are all adaptive--it can not so completely account for those constituting species; for these consist mostly of _trivial_ differences in coloration, relative proportion of parts, which are of _no perceivable use_ in the struggle for life, and therefore could not be preserved and integrated by natural selection. Therefore, according to Romanes, natural selection is a theory of origin of adaptive structures rather than of origin of species. Comparing to a growing tree, once admit lateral buds started, and natural selection completely accounts for the growth in different directions, and therefore for the profuse ramification; but the origin of the lateral buds is not explained.