Modification and Variation. 319 



cases has use-inheritance prevailed. But it appears to me 

 that, on the lines above sketched out, we may accept the 

 facts adduced by the transmissionist, and at the same time 

 interpret them on selectionist principles. 



It may be well now briefly to summarize the line of 

 argument in a series of numbered paragraphs. 



1. In addition to what is congenitally definite in 

 structure or mode of response, an organism inherits a 

 certain amount of innate plasticity. 



2. Natural selection secures 



(a) Such congenital definiteness as is advan- 



tageous. 



(b) Such innate plasticity as is advantageous. 



3. Both a and b are commonly present ; but uniform 

 conditions tend to emphasize the former; variable con- 

 ditions, the latter. 



4. The organism is subject to 



(a) Variation, of germinal origin. 

 (/3) Modification, of environmental origin, affect- 

 ing the soma or body tissues. 



5. Transmissionists contend that acquired somatic modi- 

 fication in a given direction in one generation is transmitted 

 to the reproductive cells to constitute a source of germinal 

 variation in the same direction in the next generation. 



6. It is here suggested that persistent modification 

 through many generations, though not transmitted to the 

 germ, nevertheless affords the opportunity for germinal 

 variation of like nature. 



7. Under constant conditions of life, though variations 

 in many directions are occurring in the organisms which 

 have reached harmonious adjustment to the environment, 

 yet natural selection eliminates all those which are dis- 

 advantageous, and thus represses all variations within 

 narrow limits. 



