86 PRODUCTION OF NATIONALITY 



parents and their offspring comes about in two ways ; 

 there is the common hereditary material, ready in 

 parent and child to respond in the same way to 

 similar environmental forces, and there is the fact 

 that the forces of the environment acting on parent 

 and child are usually alike. We must realize that 

 the environmental forces are as necessary to the final 

 result as is the initial material, but we are apt to 

 neglect them because of their constancy. As I have 

 already said, it is impossible to separate the selective 

 from the plastic effects of the environment, and when 

 similar conditions have persisted through many 

 generations, selection must have been at work. Only 

 those strains capable of responding successfully to 

 the prevailing conditions can maintain existence, so 

 that there is a continuous and gradual elimination of 

 the other strains. Thus it has come about that all 

 life has grown old and formal with regard to the 

 persistent features of its environment, and if these 

 are not present, it fails to develop and dies. 



With regard to a much larger number of factors 

 than we are inclined to suspect, the possibility of 

 response to alternative conditions persists. Alpine 

 conditions, for example, with their combination of 

 decreased atmospheric pressure and increased humid- 

 ity, greater radiation and extremes of heat and cold, 

 cause immediate changes in all animals and plants 

 that are not at once killed by the transplantation. 

 Removal from the hills to the plains produces a 

 direct effect on plants, animals and human beings. 

 Transference to a warmer climate thins the feathers 

 of birds and the fur of mammals. The keeping of 

 monkeys out of doors, and the transference of the 



