There exists in the individual organism a duplex relation between growth and structure which it is difficult adequately to express. Excluding the cases of a few low organisms living under special conditions, we may properly say that great growth is not possible without high structure. The whole animal kingdom, throughout its invertebrate and vertebrate types, may be cited in evidence. On the other hand, among the superior organisms, and especially among those leading active lives, there is a marked tendency for completion of structure to go along with the arrest of growth. While an animal of elevated type is growing rapidly, its organs continue imperfectly developed—the bones remain partially cartilaginous, the muscles are soft, the brain lacks definiteness; and the details of structure throughout all parts are finished only after growth has ceased. Why these relations are as we find them, it is not difficult to see. That a young animal may grow, it must digest, circulate blood, breathe, excrete waste products, and so forth; to do which it must have tolerably-complete viscera, vascular system, etc. That it may eventually become able to get its own food, it has to develop gradually the needful appliances and aptitudes; to which end it must begin with limbs, and senses, and nervous system, that have considerable degrees of efficiency. But, along with every increment of growth achieved by the help of these partially-developed structures, there has to go an alteration of the structures themselves. If they were rightly adjusted to the preceding smaller size, they are wrongly adjusted to the succeeding greater size. Hence, they must be remoulded—unbuilt and rebuilt. Manifestly, therefore, in proportion as the previous building has been complete, there arises a great obstacle in the shape of unbuilding and rebuilding. The case of the bones shows us how this difficulty is met by a compromise. In the thigh-bone of a boy, for instance, there exists, between the condyle, or head, and the cylindrical part of the bone, a place where the original cartilaginous state continues; and where, by the addition of new cartilage and new osseous matter, the shaft of the bone is lengthened: the like going on in an answering place at the other end of the shaft. Complete ossification at these two places occurs only when the bone has ceased to increase in length; and, on considering what would have happened had the bone been ossified from end to end before its growth was complete, it will be seen how great an obstacle to growth is thus escaped. What holds here, holds throughout the organism: though structure up to a certain point is requisite for further growth, structure beyond that point impedes growth. How necessary is this relation we shall equally perceive in a more complex case—say, the growth of an entire limb. There is a certain size and proportion of parts, which a limb ordinarily has in relation to the rest of the body. Throw upon that limb extra function, and within moderate limits it will increase in strength and bulk. If the extra function begins early in life, the limb may be raised considerably
