CHAPTER XVIII 



INSTINCTS 



THE teleological way of analyzing animal conduct has 

 predominated to such an extent that there has been a 

 tendency to connect all animal reactions with the preser- 

 vation of the individual and the species. Instincts are 

 considered to be such reactions of the organism as a whole 

 which lead to the nutrition of the individual, the mating 

 of the two sexes, and the care of the offspring. If the 

 tropism theory of animal conduct is justified it must be 

 possible to show that instincts are tropistic reactions. 



We have insisted in previous chapters that animals 

 indifferent to light can be made strongly positively or 

 negatively heliotropic by certain chemicals or vice versa 

 (e.g., the experiments on certain fresh water crustaceans 

 with acids or alcohol and caffein) . We know that the body 

 itself produces at various periods of its existence definite 

 hormones and such hormones can act similarly as the acids 

 or the caffein in the experiments on crustaceans, since it 

 makes no difference whether such substances as acid are 

 introduced into the blood from the outside or from certain 

 tissues of the animal's own body. We know through 

 F. Lillie 's observations that in the blood of the male cattle 

 embryo substances circulate which inhibit the develop- 

 ment of secondary sexual characters of the female embryo, 

 and we know through Steinach's experiments that the in- 

 termediate tissue from the sexual gland of one sex when 

 introduced into the castrated organism of the opposite 

 sex may impart to the latter the sexual instincts of the 



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