160 TROPISMS 



Finally there are indications of the role of chemo- 

 tropism in mating. It has been observed for a long time 

 that if a female butterfly is kept hidden from sight in a 

 not too tightly closed box, male butterflies of the same 

 species will be attracted by the box and settle on it. The 

 female apparently gives off a substance to which the male 

 is positively chemotropic. All these observations should 

 be worked out more systematically. The data suffice, 

 however, to indicate that what the biologist and psycholo- 

 gist call instinct are manifestations of tropisms. 



The fact that eggs are laid by many insects on material 

 which serves as a nutritive medium for the offspring is a 

 typical instinct. An experimental analysis shows again 

 that the underlying mechanism of the instinct is a positive 

 chemotropism of the mother insect for the type of sub- 

 stance serving her as food ; and when the intensity of these 

 volatile substances is very high, i.e., when the insect is on 

 the material, the egg-laying mechanism of the fly is auto- 

 matically set into motion. Thus the common housefly 

 will deposit its eggs on decaying meat but not on fat; 

 but it will also deposit it on objects smeared over with 

 asafcetida, on which the larva? cannot live. Aseptic banana 

 flies will lay their eggs on sterile banana, although the 

 banana is only an adequate food for the larvae when yeast 

 grows on it. It seems that the female insect lays her eggs 

 on material for which she is positively chemotropic, and 

 this is generally material which she also eats. The fact 

 that such material serves as food for the coming genera- 

 tion is an accident. Considered in this way, the mystic 

 aspect of the instinctive care of insects for the future 

 generation is replaced by the simple mechanistic concep- 

 tion of a tropistic reaction. In this case natural selection 

 plays a role since species whose females would too fre- 



