APPENDIX 195 



greatly lessen, and may almost remove, natural selection so far as these par- 

 ticular adaptive qualities are concerned. A high degree of plasticity hinders 

 the development of innate qualities by selection, because it diminishes the 

 selection. Plasticity obstructs selection. As an example, think of humankind. 

 (Compare page 177.) 



This is true if we are considering innate adaptations of the same sort as 

 those produced by education. But it is not so true if we consider different 

 types of modification in the two cases. An ontogenetic modification, such as 

 a forced change in habit, leading to a change in habitat, may enable the in- 

 dividuals of a species to escape destruction. (Compare Conn's illustration 

 of the acquired arboreal habit.) Some individuals may later be born with 

 an innate taste for tree-climbing, but this would hardly give them great ad- 

 vantage over the other individuals which took to the trees generation after 

 generation because they had to, rather than because they wanted to. It is 

 doubtful if the innate instinct to climb trees would be promptly established 

 by natural selection through the extermination of the more reluctant tree- 

 climbers. The forced habit of tree-climbing would not in this case cause 

 the prompt evolution of an innate tree-climbing instinct. 



But in Conn's illustration of the acquired arboreal habit, it was not the 

 instinct of tree-climbing, but foot and hand structure suitable for climbing, 

 which were evolved. The arboreal habit adopted by the several individuals, 

 generation after generation, brought the animals into a new environment, 

 and here new structural features became advantageous and were evolved. 

 The ontogenetically acquired habit did not cause the evolution of a similar 

 innate habit, but caused the evolution of something very different, namely, 

 special foot and hand structure. The character acquired through plasticity 

 (tree-climbing habit) did not serve as a close guide to evolution, but as a 

 general influence toward the production of a different type of adaptation to 

 arboreal life. We are thus led to the conclusion that the plastic response of the 

 individual is not a close guide to the course of evolution. In a species whose 

 individuals are highly plastic, the ontogenetic modifications will usually be of 

 a different sort from the adaptive innate characters which may arise later. 



We come, then, to this general result: In a species whose members 

 are but slightly plastic, or slowly responsive to modifying influences, innate 

 characters similar to those ontogenetically acquired may be evolved; but, 



