1 86 COMPARATIVE PHYSIOLOGY OF THE HRAIN 



4. We will now turn our attention to the consider- 

 ation of some more complicated instincts. It always 

 seemed to me one of the most wonderful arrange- 

 ments in nature that, in many species, the female lays 

 her eggs in places where the newly born larvse find 

 just the kind of food they require. The fly lays its 

 eggs on decaying meat, cheese, or similar material, 

 and it is on these substances that the young larvse 

 feed. I have often placed pieces of lean meat and 

 pieces of fat from the same animal side by side on 

 the window-sill, but the fly never failed to lay its eggs 

 on the meat and not on the fat. I further tried to 

 raise the larva? on fat. As was to be expected, they 

 did not grow, but soon died. It was possible to dis- 

 cover the mechanics of the peculiar instincts of the 

 mothers through experiments on the young larvse. 

 The larvse are oriented by certain substances which 

 radiate from a centre, and this orientation takes place 

 in the same way as in the orientation of heliotropic ani- 

 mals by the light. The centre of diffusion takes the 

 place of the source of light, and the lines of diffusion 

 (that is the straight lines along which the molecules 

 move from the centre of diffusion into the surrounding 

 medium /. e., the air) the place of the rays of light. 

 The chemical effects of the diffusing molecules on 

 certain elements of the skin influence the tension of 

 the muscles, as the rays of light influence the tension 

 of the muscles in heliotropic animals. The orienta- 

 tion of an organism by diffusing molecules is termed 

 chemotropism, and we speak of positive chemotropism 



