reaction to a definite stimulus, and in the fact that a functional stimulus strengthens an organ. just as the contraction of a muscle strengthens it, so also is every other histological element better nourished when acted on by the specific stimulus to which it is adapted. The varied sensitiveness for specific stimuli here has a similar result to that which follows in the case of the individual possessing certain advantages which make it victorious in the struggle with other individuals. In whatever part of the organism a definite stimulus is at work, there will necessarily be an increase of those elements that are most susceptible to this stimulus and are excited to the highest degree of activity by it. Thus elements which are stimulated to growth and increase by tension and pressure, necessarily accumulate and arrange themselves in the direction of the stimulus in the parts where these forces act most strongly upon them. The arrangement of the spongy tissue of bones and of the complicated felting of the connective tissue in the dolphin's fin, as well as the marvellously suitable form and direction of blood-vessels, are thus to be explained; and we may in general say that a similar explanation can be given to the various delicate adaptations of the tissues of the higher animals, all of which have the power of adapting themselves to the present circumstances of the organism (see Note III, p. 57).
Roux, however, went further, in that he believed that these histological structures arose entirely by intra-selection, and not by individual selection at all. In this respect I believe he was wrong, though one cannot
