8 HEREDITY AND ENVIRONMENT 



or causes are being successfully explored. Let 

 us briefly review some of the principal events 

 in the development of animals, and particu- 

 larly of man, and then consider some of the 

 chief factors and processes of development. 

 Most of our knowledge in this field is based 

 upon a study of the development of animals 

 below man, but enough is now known of hu- 

 man development to show that in all essential 

 respects it resembles that of other animals, 

 and that the problems of heredity and differ- 

 entiation are fundamentally the same in man 

 as in other animals. 



I. Development of the Body 



The entire individual — structure and func- 

 tions, body and mind — develops as a single in- 

 divisible unity, but for the sake of clarity it is 

 desirable to deal with one aspect of the indi- 

 vidual at a time. For this reason we shall 

 consider first the development of the body, and 

 then the development of the mind. 



1. The Germ Cells. — In practically all ani- 

 mals and plants individual development begins 

 with the fertilization of a female sex cell, or 



