56 THE FOUR 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 adjustment to the same environment” (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 (1) 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. 111). ‘‘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 Princtples. 
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 codperating 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. Ina 
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) codperate. 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 codperated in producing segregate 
