The Theory of Natural Selection. 261 
external conditions of life) of almost every organic 
type alters more or less from century to century— 
whether from astronomical, geological, and geographi- 
cal changes, or from the immigrations and emigrations 
of other species living on contiguous areas, and so 
on—it follows that the process of natural selection 
need never reach a terminal phase. And forasmuch 
as natural selection may thus continue, ad infinitum, 
slowly to alter a specific type in adaptation to a 
gradually changing environment, if in any case the 
alteration thus effected is sufficient in amount to lead 
naturalists to name the result as a distinct species, 
it follows that natural selection has transmuted one 
specific type into another. Similarly, by a continuation 
of the process, specific types would become transmuted 
into generic, generic into family types,andsoon. Thus 
the process is supposed to go on throughout all the 
countless forms of life continuously and simultaneously 
—the world of organic types being thus regarded as 
in a state of perpetual, though gradual, flux. 
Now, the first thing we have to notice about this 
theory is, that in all its main elements it is merely 
a statement of observable facts. It is an observable 
fact that in all species of plants and animals a very 
much larger number of individuals are born than can 
possibly survive. Thus, for example, it has been 
calculated that if the progeny of a single pair of 
elephants—which are the slowest breeding of animals 
—were all allowed to reach maturity and propagate, 
in 750 years there would be living 19,000,000 de- 
scendants. Again, in the case of vegetables, if a 
species of annual plant produces only two seeds a 
