Page:George Archdall Reid 1896 The present evolution of man.djvu/128

116 lesser development of them all. But, of a thousand structures, some must vary in one direction and some in the other; the general result must therefore be according as the most vary, or as the most important vary; but whatever the general result, there is ever preserved a considerable degree of harmony and proportion between the structures in normal animals; in abnormal animals (i.e. in monsters, in deformed persons, and persons with tumours) the structures have not the power to vary proportionately; under stimulation either some develop more in proportion, or others less in proportion than is normal; but these animals being usually among the unfit, perish without leaving descendants.

The general effect of Natural Selection, at least as regards higher animals, therefore, is to produce in the individual organism the power of varying proportionately and harmoniously in all its parts along certain lines in response to stimulation, direct or indirect, from the environment; for instance, owing to direct stimulation the skin of the sole thickens and hardens, owing to indirect stimulation (i.e. in response to functional activity caused indirectly by events or objects in the environment), the muscles of a limb grow and strengthen. From which it follows, that the size any individual of a species attains, depends firstly on the power to vary under stimulation inborn in his structures, and secondly, on the amount of appropriate stimulation to which the structures are subjected, whereby this power to vary is called into activity; the structures in the limbs of an infant, for example, grow into an adult limb under appropriate stimulation, that of use; but if the stimulation be withheld, the limbs remain infantile, nay, if stimulation be withheld absolutely, even the infantile standard of development is not maintained, and the more active tissues, those most largely composed of