408 EVOLUTION AND GENETICS 



hereditary qualities in the germinal determiners. With our modern 

 knowledge of heredity and the theory of the gene we realize that 

 germ cells do contain the genetic foundations of the new individual 

 and that the genes are probably subject to variation like the 

 characters which they bring to expression. This does not, how- 

 ever, explain the process of change. 



Three points in Weismann's theory should be borne in mind: 

 1. The idea of determinants foreshadows the well-established 

 theory of the gene, although it was necessarily vague and imperfect 

 in detail. 2. Weismann's emphasis upon nourishment brings out 

 the importance of environment. This is all the more suggestive 

 when we consider that he did not believe in the activity of the 

 environment in evolutionary processes. 3. The idea that deter- 

 minants, although hypothetical structures, obey the same laws as 

 the larger units of living matter which can be observed and sub- 

 jected to experiment is an important and necessary interpretation 

 which we can apply profitably to the gene. 



Roux's Intraselection. Prior to the formulation of Weismann's 

 theories, Roux proposed a theory of intraselection which was 

 fundamentally similar in that it assumed struggle among parts of 

 an organism for opportunity to develop as a basis for the evolution 

 of its parts. The theory assumes that conditions within an organ- 

 ism include mechanical relationships such as contact and pressure 

 which exert an influence upon the development of the parts 

 involved. For example, pressure on a bone is supposed to result 

 in a greater deposition of bony tissue to meet the stress. Objection 

 has been made to this theory on the ground that it involves a 

 distinctly Lamarckian point of view, for cianges so induced would 

 be responses to the internal environment. Many biologists are 

 still unwilling to accept this point of view. In any case the theory 

 is of minor importance, and must be looked upon as a subsidiary 

 theory. It has not been wholly condemned, although there are 

 excellent reasons for only partial acceptance. 



Coincident Selection. This theory, variously expressed, has 

 been widely used in an attempt to utilize Lamarckian characters 

 without recourse to Lamarckian principles. Proponents of the 

 theory recognize the value of individual adaptations, but explain 

 their effect as merely the preservation of the organism until the 

 appearance of germinal variations capable of producing similar 

 adaptations in the entire species. 



