But if we must explain, through natural selection, such complex series of actions as are necessary to the making of the cocoon of the silkworm or of the Saturnia moth (_Saturnia carpini_), what reason have we for not referring other instincts also to selection, even if they be repeated several times, or often, in the course of a lifetime? It is illogical to drag in any other factor, if this one, which has been proved to operate, is sufficient for an explanation.
Thus, as far as instincts are concerned, there is no necessity to make the assumption of an inheritance of functional changes, any more than there is in regard to any purely morphological modifications. As the instincts only exercised once show us that even very complicated impulses may arise without any inheritance of habit, that is, without inheritance of functional modification, so there are among purely morphological characters not a few which, though effective, are purely passive, which are of use to the organism only through their existence, and not through any real activity, so that they cannot be referred to exercise, and therefore cannot be due to the transmission of the results of exercise. And, if this be the case, then transformations of the most diverse parts may take place without the inheritance of acquired characters, that is, of functional modifications, and there is no reason for dragging in an unproved mode of inheritance to explain a process which can quite well be explained without it. For if any part whatever can be transformed solely through natural selection, why, since there is general variability of all parts, should this be confined to the passive organs alone, when the active ones are equally variable, and equally important in the struggle for existence?
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.