In the cases of species, where some individuals develop farther than others, we say the former possess more growth-force, or “vigor,” than the latter. We may therefore say that higher types of structure possess more “vigor” than the lower. This, however, we do not know to be true, nor can we readily find means to demonstrate it. The food which is taken by an adult animal is either assimilated, to be consumed in immediate activity of some kind, or stored for future use, and the excess is rejected from the body. We have no reason to suppose that the same kind of material could be made to subserve the production of life-force by any other means than that furnished by a living animal organism. The material from which this organism is constructed is derived first from the parent, and afterward from the food, etc., assimilated by the individual itself so long as growth continues. As it is the activity of assimilation directed to a special end during this latter period which we suppose to be increased in accelerated development, the acceleration is evidently not brought about by increased facilities for obtaining the means of life which the same individual possesses as an adult. That it is not in consequence of such increased facilities possessed by its parents over those of the type preceding it, seems equally improbable when we consider that the characters in which the parent’s advance has appeared are rarely of a nature to increase those facilities. The nearest approach to an explanation that can be offered appears to be somewhat in the following direction: