314 Biology in America 



downward which is the only direction open to it where it 

 finds new young leaves on which it can feed. The wonderful 

 hereditary instinct upon which the life of the. animal depends 

 is its positive heliotropism in the unfed condition and the 

 loss of this heliotropism after having eaten. The chemical 

 changes following the taking up of the food abolish the 

 heliotropism just as CO 2 arouses positive heliotropism in 

 certain Daphnia." 6 



Such an instinct as that of this caterpillar is however a 

 relatively simple one. Can those wonderfully complex in- 

 stincts of so many animals which are connected with the pro- 

 duction and care of the young be likewise relegated to the 

 realm of the purely mechanical? To bring the reactions of 

 so complex an organism as a vertebrate animal with its highly 

 developed brain, nerves and sense organs into line with those 

 of a unicellular form or a non-nervous plant in the present 

 state of our knowledge is a matter of great difficulty. It can 

 be shown with a reasonable degree of probability however that 

 even here what we call "instinct" may be purely a response 

 to physical or chemical stimuli, modified by certain substances 

 secreted by the body and known as "hormones" from the 

 Greek verb hormao to excite. 



The role of these substances and the bearing which they 

 have on the "mechanistic conception of life" we shall dis- 

 cuss later, merely bearing in mind their existence at this 

 point, in order to appreciate what follows. ( 



In many fish, as for example the minnow Fundulus, the 

 act of mating consists in the sexes pressing their bodies close 

 together in such a way that as the eggs are laid by the fe- 

 male the sperms are pressed out by the male and are thus 

 mixed with, and fertilize the eggs in the water. That this 

 behavior on the part of the female at least is similar to a 

 response to a solid object is shown by keeping the sexes sepa- 

 rate at the spawning season, in which case the female will 

 mate with the glass wall of the aquarium, when she happens 

 to come in contact with it. This reaction is usually devel- 

 oped only in the spawning season through the influence of 

 the hormones secreted at that time, but if the female is kept 

 permanently isolated from the male she may perform this act 

 at any time of year. 



Loeb quotes the late Professor Whitman to the effect that 

 male pigeons isolated from the females will attempt to mate 

 with any solid object in their field of vision, e.g., glass bot- 

 tles, and even with objects which give only the optical im- 

 pression of a solid, namely their own shadow on the ground. 

 And Craig has shown that male pigeons under these circum- 



8 Loeb, locus citatus, pp. 161-2. 



