46 CONTROLLED NATURAL SELECTION 
The Relation of Species to Species.—The 
environment of one species may form that 
of another—beneath forest trees, tree-ferns 
thrive ; on the stems of tree-ferns, filmy ferns 
find suitable homes. The cat feeds on the 
mouse, whose enemy the cat is. Thus, one 
species can, through its environment, affect 
that of another; but a species, per se, cannot 
affect another species because action implies 
something acted upon, and anything upon 
which a species acts is its environment. 
Species may occupy a common environment, 
but, as will be shown, in so doing, neither 
each other nor their specific environments 
are affected. 
; The Species and its Environment. Their 
Relations.—So far, the following conceptions 
have been arrived at. Structure equals en- 
vironment. The boundaries of the specific 
environment of a species are defined. Each 
species has its own specific environment. A 
species completely fills its specific environ- 
ment. A species can only be acted upon 
by physical and biological forces, through its 
environment. 
Further consideration must now be made 
of the selecting agent, the Environment. It 
has been shown not to be homogeneous, but 
