Origin of Species, p. 52. 
324 
bird in the world.”* Thus the one reason given for the 
extinction of large animals, which is based on the theory 
of Natural Selection, carrot be sustained. 
We have considered the case of the transformation of 
a whole species and of the destruction of a whole species. 
But there is another possibility to be considered. Some 
members of a species may remain unmodified, while others 
may undergo transmutation in one or more directions. 
In this way, it is conceivable that there should be a bifur 
cation and even a ramification of species. If these can 
co-exist amicably, they will lead to a manifold transmu 
tation of species ; but if they compete with one another, 
this will lead either to the extermination of the unmodified 
parent form or of the nascent varieties. Let us, then, see 
what probability there is that the bifurcation or ramifi 
cation of species has been brought about by Natural 
Selection ; or what probability there is that the victory 
of species over variety, or variety over species, has been 
brought about by Natural Selection. 
We are indebted to Mr. Fiske for an elaborate picture 
of the process by which the bifurcation of a species may 
be brought about by Natural Selection. This method of 
argument is infinitely superior, in my opinion, to the 
plan which is sometimes adopted, when the action of 
a principle is dogmatically asserted but no hint is given 
as to how it might be accomplished, to say nothing of 
any evidence that it has actually been accomplished. 
Mr. Fiske says :— 
“ We may form a rough notion of the way in which a single species 
bifurcates into two well defined species. Suppose a race of ruminants 
to have been living in Africa before the introduction of carnivora, and 
suppose that for sundry reasons, the vitality of the race was but little 
affected by moderate variations in the sizes of its individuals, so that