fflSTORICAL 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 Hght 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. This inquiry strikes at the very roots of the causo- 

 mechanics of evolution, and it is essential that the attack upon this 

 problem be followed up with the utmost vigor if we are to make any 

 real progress in our analysis of the how and why of evolution. Some 

 progress has been made already, but it is the expectation of the writer 

 that the next two or three decades will be characterized by as impor- 

 tant discoveries in this field as those that have been made in the field 

 of heredity during the past two or three decades. When that problem 

 shall have been solved, it will be well to attack, in the light of these 

 results, the problem of how the genes produce or effect the develop- 

 ment of the characters of the embryo, the larva, and the adult. 

 When we know this we shall be in a position to attack some of the ulti- 

 mate problems of biology that must await the accession of such new 

 knowledge before their solution can be attempted. 



THE PRESENT ATTACK UPON EVOLUTION IN THE UNITED STATES 



The present 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, 

 but it is significant as an influence that may serve either greatly to 

 accelerate or to retard the progress of our science. The writer's own 

 experience is that the controversy has greatly enhanced popular inter- 



