54 
PBOOFS OF EVOLUTION. 
very principle of Evolution demands reversion in 
such cases. 
There are, however, two interesting questions, 
not yet fully answered; although they do not 
affect at all the truth or probability of the theory 
of Descent. First, what is the cause of variation ? 
And, secondly, What is the cause of the first step 
in usefulness f Why should there be a tendency to 
vary?* Use can improve an organ, but how can 
it start one ? And how does it start itself ? The 
answer to these questions may perhaps be found 
in subsequent investigations of the psychological 
phases of the life-problem. 
* Since the delivery of this Essay, a writer in the “Popular 
Science Monthly” for April, 1889, maintains with much force that 
the Tendency to Vary is due largely to Psycho-Physiological 
influences. 
