THEORY OF INSTINCTS 187 



when the animal is forced to bring its axis of symme- 

 try into the direction of the lines of diffusion and to 



J 



turn its head toward the centre of diffusion. In 

 such an orientation every pair of symmetrical points 

 on the surface of the animal is met by the lines of 

 diffusion at the same angle. It can easily be shown 

 that larvae of the fly are positively chemotropic toward 

 certain chemical substances which are formed, for 

 instance, in decaying meat and cheese, but which are 

 not contained in fat. The substances in question 

 are probably volatile nitrogenous compounds. The 

 young larvae are probably led by those substances to 

 the centre of diffusion in the same way as the moth 

 into the flame. The female fly possesses the same 

 positive chemotropism for these substances as the 

 larvae, and is accordingly led to the meat. As soon 

 as the fly is seated on the meat, chemical stimuli seem 

 to throw into activity the muscles of the sexual or- 

 gans, and the eggs are deposited on the meat. It 

 may also be possible that at the time when the fly is 

 ready to deposit its eggs the positive chemotropism 

 is especially strongly developed. It is only certain 

 that neither experience nor volition plays any part in 

 these processes. If the question be raised as to what 

 is necessary in order to produce these reactions, the 

 answer is, first, the presence of a substance in the skin 

 or certain parts of the skin (sense-organs) of the ani- 

 mal which is altered by the above-mentioned volatile 

 substances contained in the decaying meat, and 

 second, the bilateral symmetry of the body. The 



