478 HEREDITY AND ENVIRONMENT 



which nothing can change. Development is 

 not a reversible process; a man cannot enter 

 a second time into his mother's womb and be 

 born again. Once the sex cells are formed 

 their hereditary nature is determined ; once the 

 egg is fertilized the hereditary possibilities of 

 the new individual are fixed ; once any stage of 

 development has passed that page in the book 

 of life is closed and sealed. 



And yet at every step in this long process of 

 development there were one or more alterna- 

 tives which might have been taken instead of 

 the one which was taken. There were innu- 

 merable possible alternatives in the matings of 

 our ancestors, there were billions of possible 

 alternatives in the union of the millions of 

 types of germ cells which each of our parents 

 produced; at every step in the development of 

 the oosperm from which each of us came there 

 were many possible alternative stimuli and re- 

 sponses. But in each case one of these in- 

 numerable alternatives was taken and the oth- 

 ers left. In every instance there was some 

 cause that determined which alternative was 

 taken, but these causes are so local and indi- 



