Page:The Effect of External Influences upon Development.djvu/25

Rh very different primary constituents of corresponding parts, derived from both father and mother, must meet in the germ.

As a rule the parts of the adult parents are so different that they could not be interchanged without the formation of monsters; and similarly the primary constituents of their germ-substance could not be united together to produce a young organism, exhibiting harmony in its various parts, if they did not all have a certain scope for variation, so as to render them capable of adaptation to one another. And though we know so little as yet of these hidden processes, we can hardly err in thinking that relations are here involved which are included in the idea of intra-selection. The struggle of parts unequal in strength—that is, unequal in susceptibility to stimulus—must be the cause which brings about the mingling of parental primary constituents, and prevents the occurrence of monsters with parts ill adapted to one another.

Intra-selection apparently also renders it possible for a harmonious organism to live and undergo development from an only moderately harmonious mass of primary constituents, such as I imagine constitute the germ-plasm. But the supposition of the whole activity of intra-selection presupposes the specific sensitiveness of the various primary constituents and of the units of smaller or larger groups of these; and this sensitiveness can naturally only have arisen through ordinary selection of individuals, owing to variation of the germ. For it is hereditary in this case just as in that of plants, the sensibility of which—(geotropic, heliotropic,