DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 1 07 



environment or from other causes, a sudden and considerable 

 change from the parent type. Such a product is known as a 

 "sport." It is quite possible that natural selection may seize 

 on such and if in a favorable direction preserve and increase 

 them. In such cases adaptation might take place with great 

 rapidity, instead of in the gradual way described above. 



In the second place 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, very 

 great 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 progressive 

 adaptation of animals to the environment. There is no rea- 

 sonable doubt that natural selection is such an explanation. 

 So far as we know either small or large variations may be se- 

 lected if they are only inherited variations and useful ones. 

 To what extent it is assisted by other factors is at present un- 

 certain. It will be assumed in the following pages that it is one 

 of the most important known factors in producing adaptation. 



141. 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 



