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stances favour some races, they begin to increase in 
numbers to an enormous extent; they are as prosperous as 
it is possible for them to be, but their prosperity is fatal to 
many of them as individuals, and sometimes threatens their 
existence as a race. Enemies of all kinds are attracted to 
the unwonted feast, and the happy creatures which a short 
time ago rejoiced in their numbers, are threatened with 
extinction. This is surely a struggle for existence, but 
a struggle which may leave, and actually has left, the race 
altogether unmodified in structure, as in the case of the 
mice in La Plata, described by Mr. Hudson—already 
quoted on page 54. 
In considering the bearing of stability of species upon 
the theory of Natural Selection, it is first of all obvious 
that it limits the power of Natural Selection considered 
as an agent in the transmutation of species. Enormous 
numbers of living beings have existed generation after 
generation through the aeons of the geological ages, and 
their specific types have remained unaltered. Such stability 
has prevented the transmutation of species by whatever 
means it might otherwise have been effected. But this is 
not all. It is quite conceivable that, owing to causes which 
will be referred to shortly, some individuals of an otherwise 
unchanged species might become modified so as to be 
transmuted into a new species. But so long as circum 
stances remained unaltered, no such change could be 
wrought by Natural Selection ; for so long as the con 
ditions remained unchanged, the organisms which had 
once been adapted to those conditions would not gain any 
advantage from any such modification, and therefore 
no such change could be brought about by Natural 
Selection. 
One might have supposed that the facts which have 
just been cited would have led the advocates of the theory