Page:The Effect of External Influences upon Development.djvu/15

Rh while others, such as the climbing shoots of the ivy and pumpkin, turn away from it, or are 'negatively heliotropic.’ As these differences are highly purposeful—inasmuch as in one case they put the plant in a position to make the utmost use of the light, and in the other enable it to climb—we must look upon the differences in structure which cause them as due to adaptation. Once more therefore we have no other explanation of their origin than that offered by selection (see Note II, p. 56).

In all these instances we have to deal with hereditary structures, that is with arrangements which always develop in the same manner under the ordinary conditions of life and growth, and which in their turn regulate the plants, so that they respond in a fitting manner to external stimuli.

Similar arrangements also exist in animals, and play an important part in their development.

Hermann Meyer seems to have been the first to call attention to the adaptiveness as regards minute structure in animal tissues, which is most strikingly exhibited in the architecture of the spongy substance of the long bones in the higher vertebrates. This substance is arranged on a similar mechanical principle to that of arched structures in general: it is composed of numerous fine bony plates so arranged as to withstand the greatest amount of tension and pressure, and to give the utmost firmness with a minimum expenditure of material. But the direction, position, and strength of these bony plates are by no means innate or determined in advance: they depend on circumstances. If