Page:The Effect of External Influences upon Development.djvu/20

16 and the shoot upwards, while the branches pass obliquely outwards. The sensitiveness—the positive or negative geotropism—is inherited, and depends on the nature of the germ; but the special direction which the growing part takes is a consequence of the varying conditions of existence of the individual; it is acquired anew in every individual life, and cannot be transmitted. The great significance of intra-selection appears to me not to depend on its producing structures that are directly transmissible,—it cannot do that,—but rather consists in its causing a development of the germ-structure, acquired by the selection of individuals, which will be suitable to varying conditions. Intra-selection effects the special adaptation of the tissues to special conditions of development in each individual. When a tree is crowded by other trees on one side, and is thus cut off from the light, it grows less rapidly on this side, but develops all the more luxuriantly on the other: the hereditary so-called 'molecular' constitution of its shoots, which we speak of as positive heliotropism, is the cause of this. Similarly, when a fractured bone heals out of the straight, the plates of the spongy portion again become set in the line of greatest tension and pressure, the cause, being the hereditary molecular sensitiveness of the connective tissue-matrix of the bone.

We may therefore say that intra-selection effects the adaptation of the individual to its change developmental conditions,—the suiting of the hereditary primary constituents to fresh circumstances. But these primary constituents themselves could only be produced by