56 THE POUR SEGREGATIVE PRINCIPLES. 



lated to each other, but quite distinct from the mother species from 

 which they have been separated by many successive isolations. 



Mr. Headley recognizes that there may be change in the species 

 without change in the environment in what he says concerning "alter- 

 native methods of adjust-ment to the same environinent" (p. 149). He 

 illustrates this in the following way : ' ' For many butterflies, birds [that 

 prey upon them] are an important part of the environment. They 

 may adjust themselves to it (i) by developing great powers of flight, 

 or (2) an offensive taste, or (3) they may 'mimic' another nauseous 

 species, or (4) come to resemble some such thing as a dead leaf and so 

 be protectively colored" (pp. 147, 148). This power which is here 

 recognized by Mr. Headley as belonging to many species fully justifies 

 his statement that ' ' Often there is a variety of possible adaptations to 

 one and the same environment" (p. 146). How, then, can he defend 

 the following sweeping generalization given by him in many passages, 

 on widely separated pages? I will quote only two or three. "If the 

 environment remains unaltered, evolution ceases. * * * Further 

 evolution can take place only if an environment that is in some way 

 different offers itself" (p. 103). "Each successive step of evolution is 

 due to some change in the environment" (p. iii). "Nothing but 

 change of environment can lead to further evolution" (p. 153). 



Notwithstanding this inconsistency in his interpretations, he gives 

 us many fresh and interesting facts. 



2. Importance of Each of the Principles. 



It is not easy to determine whether each and all of these four prin- 

 ciples began to control evolution from the very dawn of life ; but when 

 once fully established we find them cooperating in such a way that it 

 seems impossible to overestimate the importance of each in its own 

 sphere of action. It is now generally recognized that monotypic evo- 

 lution, that is, the transformation of any single type into a succession 

 of forms through which it gradually passes without branching, is 

 largely due to selection; but, in a world where two or more species 

 exist, even in the case of monotypic evolution, the whole process is 

 also dependent on isolation ; for if the single type under consideration 

 freely crosses with other types, it loses its separate identity. In a 

 world of diverse types the monotypic evolution of any one type is, 

 therefore, necessarily a process of intensive segregation, in which iso- 

 lation and the principles producing transformation (of which selection 

 and election are the chief) cooperate. Since sexual reproduction 

 began heredity has been controlled by the segregate generation of 

 individuals according to their innate aptitudes for adaptive response ; 

 and isolation and selection have cooperated in producing segregate 



