202 Heredity and Eugenics 



presence, character, or intensity, in relation to the organisms, 

 and to these forces and relations are attributed a variable 

 value in the production of germinal variations. All kinds 

 of extreme demands are made upon external forces, from 

 the conception of an organism as a plastic material which is 

 pressed into shape, and given its characters by the stress 

 of environment, to the opposite assertion that environ- 

 ment acts, if at all, as a minor factor in eliminating the unfit. 

 From logic and argument no truth may be expected, and 

 the only hope of progress in the quest for truth in this 

 problem lies in the domain of exact genetic research. 



In this chapter which is a summary of recent advances 

 and not a historical resume of the whole subject the earlier 

 work is not discussed, because it has been so often summar- 

 ized that good discussions of it are available in many 

 publications. All of the older work, however, is seriously 

 defective when considered from the viewpoint of present- 

 day genetic investigations. 



IN PLANTS 



In the bacteria and yeasts the refined and accurate 

 methods of investigation now used, and the fuller recog- 

 nition of the genetic requirements have made possible studies 

 which have given much valuable information. The work 

 of Pringsheim, Winogradsky, Hansen, Barber, Beijerinck, 

 Buchanan, and others stands as examples of what may be 

 accomplished in the study of this problem in these simple 

 organisms. In all of the observations thus far made upon 

 these organisms response to incident forces or changed con- 

 ditions is immediate, and departures, often of considerable 

 magnitude in form and function, occur; but in most in- 

 stances difficulty in fixing these modified characters has 

 been encountered. 



