There are, indeed, many of these passive parts among animals; I need only recall the coloration of animals, the whole set of skeletal parts, so diversely formed, of the Arthropods, the legs, wings, antennæ, spines, hairs, claws, and so on, none of which can be changed by the inherited results of exercise, because they are no longer capable of modification by exercise; they are ready before they are used; they come into use only after they have been hardened by exposure to the air, and are no longer plastic; they are at most capable of being used up or mutilated. Finally, even so convinced an advocate of the Lamarckian principle as Herbert Spencer has stated that among plants the great majority of characters and distinctive features cannot be explained by it, but only through the principle of selection; all the diverse protective arrangements of individual parts, like thorns, bristles, hairs, the felt-hairs of certain leaves, the shells of nuts, the fat and oil in seeds, the varied arrangements for the dispersal of seeds, and so on, all operate by their presence alone, not through any real activity which causes them to vary, and the results of which might be transmitted. An acacia covered all over with thorns seldom requires to use its weapons even once, and if a hungry ruminant does prick itself on the thorns it is only a few of these which are thus 'exercised,' the rest remain untouched. But since all these parts have originated notwithstanding their passivity, there must be a principle which evokes them in relation to the necessities of the conditions of life, and this can only be natural selection, that is, the self-regulation of variations in reference to utility. And if there is this principle, we require no other to explain what is already explained. I can quite well understand, however, that many naturalists, and especially palæontologists, find it difficult to accept this conclusion. If we think only of those parts that actively function, and thus change by reason of their function, being strengthened by use and weakened and diminished in size by disuse, and if, further, we follow these parts through the history of whole geological epochs, we may certainly get the impression that the exercise of the parts has directly caused their phyletic evolution. The direction prescribed by utility in the course of the individual life and in the phylogeny is the same, and the intra-selection, that is, the selection of tissues within the individual animal, leads towards the same improvements as the selection of 'persons.' Thus it appears as if the phyletic variations followed those of the individual life, while in reality the reverse is true; the changes arising from variations in the _germ are primary, and they determine the course of phylogenesis_, while the tissue-selection in the individual life only elaborates and improves, according to the demands made upon it, the material afforded by the primordial equipment of the germ.