THE SELECTING POWER OF ENEMIES 51 
tion or subtraction of one or more factors 
to or from the specific environment. 
The subtraction of a factor may lead to 
extinction of the species; a food supply may 
fail. Or if the species continues to thrive, 
then those structures which were related to 
the lost factor will degenerate. Those indi- 
viduals who waste the least nourishment or 
sustenance on a useless organ will have an 
advantage over others in which this organ 
is more developed. Degenerated organs are 
found in species occupying degenerated en- 
vironments. Degenerate structures will be 
found to vary most because of the absence of 
the fixing power of environmental selection. 
If a factor be subtracted locally, then a 
local degenerate variety or sub-species will 
be found. 
The addition of a new factor to the specific 
environments may produce extinction. Short 
of extinction, an added factor will prevent 
a species fitting its environment perfectly ; 
but sooner or later, as the result of variation 
of the species and environmental selection, 
new structures will arise which will enable 
the species to perfectly fit the new environ- 
ment. Further addition of factors will be 
followed by the addition of new structures ; 
