GROWTH AND REPRODUCTION 51 



II 



While we canot fail to recognize that it is an im- 

 mense gain to biological science to know that the 

 chromosomes may be the effective agents hi the 

 determination of sex and may give us a definite 

 mechanistic explanation of Mendelian inheritance, 

 yet we should not deceive ourselves as to the magni- 

 tude of the task which is still before us, or the limi- 

 tations of our actual knowledge of heredity. If 

 we ask ourselves how the chromosomes act as the 

 carriers of hereditary qualities, we are at once con- 

 fronted by grave difficulties. Let us take, for 

 example, the problem of the inheritance of that 

 racial memory which we call instinct. We know 

 that as soon as a chick leaves its shell it begins to 

 peck in search of food. This action is instinctive 

 in the sense that it is not based on any experience of 

 the individual chick, and has been conceived by 

 many biologists as due to a kind of memory of the 

 experiences of preceding generations. This memory 

 can only be conceived as corresponding to some 

 sort of physical traces in the protoplasm of those 

 cells which were destined to become the nervous 

 system of the chick, traces established there as the 

 result of the formation of certain habits of action 

 in the ancestors. Now, assuming such traces to 

 lie at the foundation of instinct (and also at the 

 foundation of other properties connected with the 



