254 Animal Instincts and Tropisms 



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. 



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 spermatozoon 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 physico- 

 chemical explanation; for example the curious experi- 

 ment 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. 



Marchal's observations on the laying of eggs by the 

 naturally sterile worker wasps are a similar case. The 

 fact that such workers lay eggs when the queen is 

 removed or when they are taken away from the larvae 



