HISTORICAL ACCOUNT OF EVOLUTION THEORY 45 
mechanism at the time of fertilization may or may not realize its 
normal somatic differentiation, depending 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 the Greeks to Darwin (The Macmillan Company). 
