CHAPTER III 
THE UNDERLYING IDEA 
WE have seen in the preceding chapters how the 
idea of evolution worked its way through the minds 
of men. Man after man got a glimpse of the idea, 
even among the ancient philosophers. But no one 
could speak convincingly on the subject before modern 
times, when a wider acquaintance with the animal 
world gave a body of facts on which it was safe to 
base conclusions. Even then the idea eluded men, 
until there came a worker trained by a long voyage 
around the world in which he had nothing to do ex- 
cept to study nature. He finally gathered in his mind 
material sufficient to convince himself not only of the 
truth of evolution but of the process by which this 
evolution was brought about. Every scientific prin- 
ciple is simple in its basal idea. In actual life the 
action of the principle may be so bound up with 
others as to need a skillful mind for its detection. But 
under all the complexities and modifications, like a 
silver thread woven into a cloth, runs the basal idea. 
Until a master has detected it the presence of it may 
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