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. 



THE EXPERIMENTAL INDUCTION OF HEREDITARY VARIATIONS 



With the problem of the mechanism of the heredity of individual 

 differences solved, at least in its more important essentials, attention 

 has gradually shifted to the problem as to how individual differences 

 arise. They seem to arise suddenly and as though of their own accord, 

 and the study of their heredity does not throw much light on the prob- 

 lem of their origin. At the present time a massed attack is being made 

 upon the problem of the mode or modes of origin of new hereditary 

 characters. The most striking success in the artificial induction of 

 mutations has been obtained by H. F. Midler, using as his material the 

 already intensively studied fruit fly, Drosophila melanogaster. By the 

 use of rather heavy doses of X-rays he succeeded in increasing the fre- 

 quency of mutations about 1,500 per cent. Nearly all of the mutations 

 produced by this method were the same as those that occur spontane- 

 ously, but many more occur in a given time. 



Many other investigators, following Muller's methods, have suc- 

 ceeded in greatly hastening the pace of mutation in various animals 

 and plants. The ability to produce such large numbers of mutants at 

 will furnishes abundant material for genetic investigation and promises 

 greatly to increase our knowledge of the intimate details of the mechan- 

 isms of variation and heredity. 



The most pressing problem of the present is that of discovering 

 how and when genes act during the course of development in producing 

 the characters of the organism. Some progress has been made in this 

 direction. 



THE RECENT ATTACK UPON EVOLUTION IN THE UNITED STATES 



The recent highly advertised attack upon the validity of the 

 principle of evolution by certain individuals and religious bodies is 

 hardly to be considered as forming a part of the history of the science, 



