68 THE PROBLEMS OF EVOLUTION 



such, but they are developed by each individual 

 in response to conditions which its body is nor- 

 mally able to produce in spite of moderate fluc- 

 tuations of the external environment. They are 

 fundamentally in contrast with the modifications 

 constituting purely individual responses not in 

 heritability hut in the source of the stimuli which 

 bring them to expression. 



The question of change in species is different 

 from that of change in individuals and is closely 

 linked with the selective processes whereby a de- 

 gree of evolution is known to be accomplished. 

 While the individual is a unit made up of insepa- 

 rable components, all necessary to its existence, the 

 species is made up of a large number of individuals, 

 varying according to recombinations of hereditary 

 units, and continues to exist even though a large 

 part of these individuals are destroyed. Change of 

 a species may result, therefore, from any factor 

 which tends to preserve one aggregation within it 

 or to destroy another. 



The individuals of a species spread in the course 

 of time over great areas. As they spread they en- 

 counter different conditions of environment, but 

 even barring this condition it is improbable that 

 they will remain exactly the same in two separate 

 regions, because of the reassortment of hereditary 

 units in sexual reproduction. Through chance or 

 through some peculiar fitness of a given heredity 



