Even though it may be difficult to think out in detail the process of the gradual origin of such an instinctive activity, exercised once in a lifetime, such as that, for instance, which impels the caterpillar to spin its intricate cocoon, which it makes only once, without ever having seen one, and thus without being able to imitate the actions which produce it, we must not push aside the only conceivable solution of the problem on that account, for then we should have to renounce all hope of a scientific interpretation of the phenomenon. We may ask, however, whether there is not something lacking in our present conception of natural selection, and how it comes about that useful variations always crop up and are able to increase. 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?