The application of our modern knowledge of heredity to human affairs has been discussed in a very original way by Bateson in his address before the British Association in Sydney to which the reader may be referred.[216] [216] Bateson, W., _Nature_, 1916, xciii., 674. CHAPTER X ANIMAL INSTINCTS AND TROPISMS[217] 1. The idea that the organism as a whole cannot be explained from a physicochemical viewpoint rests most strongly on the existence of animal instincts and will. Many of the instinctive actions are “purposeful,” _i. e._, assisting to preserve the individual and the race. This again suggests “design” and a designing “force,” which we do not find in the realm of physics. We must remember, however, that there was a time when the same “purposefulness” was believed to exist in the cosmos where everything seemed to turn literally and metaphorically around the earth, the abode of man. In the latter case, the anthropo- or geocentric view came to an end when it was shown that the motions of the planets were regulated by Newton’s law and that there was no room left for the activities of a guiding power. Likewise, in the realm of instincts when it can be shown that these instincts may be reduced to elementary physicochemical laws the assumption of design becomes superfluous. [217] Ideas similar to those expressed in this chapter may be found in the writer’s former book _Comparative Physiology of the Brain and Comparative Psychology_, New York, 1900, and in the books by George Bohn, _La Naissance de l’Intelligence_, Paris, 1909, and _La nouvelle Psychologie animale_, Paris, 1911. If we look at the animal instincts purely as observers we might well get the impression that they cannot be explained in mechanistic terms. We need only consider what mysticism apparently surrounds all those instincts by which the two sexes are brought together and by which the entrance of the spermatozoön into the egg is secured; or the remarkable instincts which result in providing food and shelter for the young generation. We have already had occasion to record some cases of instincts which suggest the possibility of physicochemical explanation; for example the curious experiment of Steinach on the reversal of the sexual instincts of the male whose testes had been exchanged for ovaries. There is little doubt that in this case the sexual activities of each sex are determined by specific substances formed in the interstitial tissue of the ovary and testes. The chemical isolation of the active substances and an investigation of their action upon the various parts of the body would seem to promise further progress along this line.