100 ZOOLOGY. 



type. Such a product is known as a " sport." It is quite pos- 

 sible that natural selection may seize on such and if in a favor- 

 able direction preserve and increase them. In such cases adap- 

 tation might take place with great rapidity, instead of in the 

 gradual way described above. 



As contrasted with natural selection of indefinite variations, 

 it has been argued that the immediate effect of the environment 

 on the organism and the efforts of the organism to respond to 

 the stimuli of the environment produce in the organism just 

 such definite variations as will tend to fit it for its surroundings. 

 In other words the majority of the variations brought about 

 by a given external condition are definite and naturally in the 

 direction to meet the necessities of the case. For example, cold 

 stimulates the surface cells of the body of an animal. The 

 immediate response of the nervous and nutritive processes in 

 the organism are such that the surface cells take on greater 

 activity and produce materials at the surface of the body 

 which tend to protect the animal from the ill effects of the cold. 

 This is an individual variation. To become effective in making 

 the species better adapted to the environment these results 

 must be handed down by inheritance to the next generation. 

 If this can take place this theory would go a long way toward 

 explaining how adaptations arise. There is, however, con- 

 siderable doubt whether such adaptations acquired in the life 

 of an individual can be transmitted to offspring. If this cannot 

 occur we are thrown back upon natural selection as the prin- 

 cipal explanation thus far offered to account for the pro- 

 gressive adaptation of animals to the environment. There is 

 no reasonable doubt that natural selection is such an explana- 

 tion. To what extent it is assisted by other factors is at pres- 

 ent uncertain. It will be assumed in the following pages that 

 it is the most important known factor in producing adaptation. 



137. Classification. Since the environment is not the same 

 at any two places on the earth and there is an accumulation, 

 from generation to generation in animals, of those features 

 which tend to bring them into harmony with their different 



