CHAPTER VIL. 
THE THEORY OF NATURAL SELECTION. 
THus far we have been considering the main 
evidences of organic evolution considered as a fact. 
We now enter a new field, namely, the evidences 
which thus far have been brought to light touching 
the causes of organic evolution considered as a pro- 
cess. 
As was pointed out in the opening chapter, this is 
obviously the methodical course to follow: we must 
have some reasonable assurance that a fact is a fact 
before we endeavour to explain it. Nevertheless, it 
is not necessary that we should actually demonstrate 
a fact to be a fact before we endeavour to explain it. 
Even if we have but a reasonable presumption as to 
its probability, we may find it well worth while to con- 
sider its explanation; for by so doing we may obtain 
additional evidence of the fact itself. And this because, 
if it really is a fact, and if we hit upon the right 
explanation of it, by proving the explanation probable, 
we may thereby greatly increase our evidence of the fact. 
In the very case before us, for example, the evidence 
of evolution as a fact has from the first been largely 
derived from testing Darwin’s theory concerning its 
method. It was this theoretical explanation of its 
