As there have been only two theories in the field, to 
disprove the one was to establish at least the strong 
probability for the other. 
There is, however, another kind of argument which has 
been brought to bear:—Looking upon the organic world 
as a product of the past, studying as best we can the 
history of the changes on the earth’s surface, we arrive 
at the conclusion that the organic world is just what we 
should expect it to be, if it were the result of the principle 
of evolution. 
So far as the first line of argument is concerned, we may 
venture to say that the strong reasons which men supposed 
they had for believing in the fixity of species have been 
overcome by the still stronger evidence in favour of the 
transmutation of species. But that is not all. It is the 
opinion of the overwhelming majority of scientific men 
that the main, if not sole, agent in this transmutation is 
the principle of Natural Selection. We have now to 
consider how far this assertion can be sustained. 
At the outset, it is of the greatest importance that we 
should clearly understand what we mean by Natural 
Selection, and realise that the phrase implies a very com 
plex idea. But in order to understand what Natural 
Selection is we must first get a clear idea of what we 
mean by “ Selection.” Why is this principle invoked, 
and what is it able to accomplish ? The answer is, that, 
in order that a race should undergo a permanent change 
equivalent to the transmutation of one species into another, 
it is necessary that variations should arise, that these 
variations should be inherited by the offspring, and that 
they should be continuously inherited until they become 
fixed in the race. Now the nature of this process will 
very much depend upon the nature of the variations acted 
on. The principle of selection is indispensable only in