22 HEREDITY AND ENVIRONMENT 



ment such as thermal, chemical and electrical 

 changes it is, in short, a distinct living thing, 

 an individuality. Under proper environ- 

 mental conditions this fertilized egg cell de- 

 velops, step by step, without the addition of 

 anything from the outside except food, water, 

 oxygen, and such other raw materials as 

 are necessary to the life of any adult animal, 

 into the immensely complex body of a star-fish, 

 a frog, or a man. At the same time, from the 

 relatively simple reactions and activities of the 

 fertilized egg there develop, step by step, 

 without the addition of anything from without 

 except raw materials and environmental 

 stimuli, the multifarious activities, reactions, 

 instincts, habits, and intelligence of the ma- 

 ture animal. 



Is not this miracle of development more 

 wonderful than any possible miracle of cre- 

 ation? And yet as one watches this marvellous 

 process by which the fertilized egg grows into 

 the embryo, and this into the adult, each step 

 appears relatively simple, each perceptible 

 change is minute; but the changes are in- 

 numerable and unceasing and in the end they 



