542 THE AMERICAN JOURNAL OF SOCIOLOGY
but we have no reason to suppose that they would not grow equally well if he did not use them. But after birth other of his structures develop little, if at all, except under the added influence of use; for example, his limbs, his heart, and his brain. If no strain were placed on these structures, they would grow little, if at all. Food alone is not sufficient for their continued growth. Lastly, if the individual be injured at any time, as by a cut, the stimulus of injury causes growth to take place during the process of healing. In man the development which results from injury is of infinitely less importance than that which results from nutri- tion, and from nutrition plus use.
Now, since no parts can be used or injured unless they first exist, and so are capable of being used and injured, nutrition must always play the first, and generally the principal, part in the development of living beings. And, moreover, when living beings first came into existence, it must have played the sole part in their development, until subsequently there was evolved the power of growing under the stimulus of use and injury.
The power of growing and developing under the influence of use is apparently quite a late product of evolution. It seems quite absent except in the higher animals, and is present to the greatest extent only in the highest. Thus an adult man owes the greater part of his bulk to growth made under the influence of use; but there is not the least evidence that most insects, for example, owe any part of their growth to its influence. They grow, as the infant grows before birth, under the sole influence of nutrition. Indeed, the most of their structural changes occur when they are quiescent undergoing metamorphosis ; that is, when they are not using the growing part of their structures. But it is not body but mind that supplies the clearest evidence that the capacity of developing under the influence of use is a late and a high product of evolution. This is easily seen when we contrast the mental development of a typical insect with that of man.
A certain beetle (Sitaris) lays its eggs at the entrance of the galleries excavated by a kind of bee (Anthophora), each gallery leading to a cell. The young larvae are hatched as active little insects, with six legs, two long antennae, and four eyes, very different from the larvae of other beetles. They emerge
