484 
whether and how often among 1,000, 100,000, or millions of indi 
viduals there is a variation which gives verdict over life and death, 
but much more that we can never determine is required ; for instance, 
the number of individuals of a species living at one time, the degree 
of their mingling with one another in their own domain, and the 
percentage occurrence of the variation in question—all which, I am 
convinced, cannot be ascertained ; and so we shall never be able to 
establish by observation the progress of Natural Selection. What is 
it, then, that nevertheless makes us believe in this progress as actual, 
and leads us to ascribe such extraordinary importance to it? Nothing 
but the power of logic ; we must assume Natural Selection to be the 
principle of explanation of the metamorphoses, because all other 
apparent principles of explanation fail us, and it is inconceivable that 
there could be yet another capable of explaining the adaptations 
of organisms without assuming the help of a principle of design. In 
other words, it is the only conceivable natural explanation of organ 
isms regarded as adaptations to conditions—(The Contemporary 
Review, vol. xliv., pp. 327-8.) 
It is a startling thing to find it asserted that, in a 
certain case to which it is applied, the theory of Natural 
Selection cannot be imagined, either with regard to the 
selective process or the morphological results. That can 
not mean that it could not be painted by the imagination 
if it were left unrestrained, for who shall put a limit to 
the creations of the fancy? It must mean that we cannot 
imagine it, because we have no grounds for believing in its 
possibility—because we know too much to be deluded, 
or because we know too little to have confidence in an 
attempt to depict what we still believe a reality. 
On the other hand, it is asserted that the statement that 
certain phenomena are the results of Natural Selection 
cannot be proved from actual observation. What cannot 
be proved by observation is simply a matter of personal 
faith, it cannot be regarded as a demonstrated law of 
nature. I venture to say that the secrets of nature have 
only been revealed to those who have had imagination to 
conceive a theory, and who have rigorously tested that 
theory by observation and experiment. But here we have