HISTORICAL ACCOUNT OF EVOLUTION THEORY 45 



mechanism at the time of fertilization may or may not realize its 

 normal somatic differentiation, de[)ending upon the presence or 

 absence of the proper environment. Cases are on record in which an 

 individual germinally determined as a female may be caused to 

 develop the secondary sexual characters of the male, or even to pro- 

 duce sperms instead of eggs. A great deal of extremely interesting 

 work on sex control and sex reversals has been done within the last 

 half-dozen years and new discoveries are being made almost daily. In 

 fact, it might be said that the genetic study of sex marks the high-tide 

 level of modern genetic advance. 



CONCLUDING REMARKS 



Now that we have traced the evolution of the science of organic 

 evolution from its crude beginnings among the Greeks up to the 

 present, we are in a position to go back and make a systematic study 

 of some of the more important phases of evolutionary science. 

 Charles Darwin found it necessary to prove the fact of organic evolu- 

 tion before attempting to discover its causes. His method of proof 

 was to marshal a great array of facts which agree with the idea of 

 descent with modification; and we shall follow Darwin's method in 

 the subsequent chapters dealing with the evidences of evolution. 



Note. — In the first half of the present historical account many short passages 

 are presented in quotation marks without mentioning the source of the quotation. 

 In all such cases it will be understood that these passages are from H. F. Osborn's 

 book, From tlie Greeks to Darmti (The Macmillan Company). 



