192 



APPENDIX 



He says with Wilhelm Roux that just as animals contend with other animals 

 for food, so the organs in the body of any one animal contend with each other 

 for food, each taking what it can get, the stronger organs (nutritionally) 

 getting most, the weaker faring more poorly. He carries this principle even 

 further and says that the parts of a single cell are engaged in a similar rivalry 

 for food and that in the germ cells the determinants thus struggle with each 

 other for nutrition. Finally he suggests that when any determinant acquires 

 an advantage in this contest for food its success will give it added vigor, en- 

 abling it to become a still more successful rival to its neighboring determinants. 

 The effect will be cumulative and generation after generation the favored 

 determinants will continue to increase in vigor. Now, as each determinant 

 gives rise to some particular portion of the adult, that part of the adult will 

 be modified step by step as its determinant becomes more favored. The effect 

 upon the favored determinant in the germ tends to be cumulative, its success 

 increasing the more vigorous it becomes, and similarly the modification of the 

 adult will steadily increase. In this way, Weismann believes, the suggested 

 trends in evolution have arisen and persisted. 



The theory is not so fanciful as this bald statement would make it seem. 

 It is certainly well worth consideration from any one who has genuine interest 

 in evolutionary problems. 



PLASTICITY, "ORGANIC SELECTION" 



If such trends in evolution exist, they suggest an interesting consideration 

 in connection with the plasticity of the individuals of certain species. We 

 have already seen (pages 27, 28, and 177) that the ability of organisms to adapt 

 themselves during their lifetime to conditions of disadvantage may enable 

 them partially to escape from the stress of the struggle for existence and per- 

 sist when, if less plastic, they would be destroyed. I cannot quite agree 

 with Morgan, Osborn, and Baldwin in the emphasis they have laid upon 

 this accommodation of the individual as a guide to the course of evolution 

 by natural selection. But if it be true that trends to evolution in particular 

 directions occasionally arise in certain species, it is conceivable that the adapta- 

 bility of the individual members of a species might tide the species over a period 

 of disadvantageous environmental conditions, giving time for some new and 



