ts. But at forty Darwin quite by
accident discovered that these things had not attracted him for
years,--that every increment of his time and energy was concentrated in
a constantly increasing measure upon the unraveling of that great
problem to which he had set himself. And he lamented bitterly the loss
of these other interests; he wondered why he had been so thoughtless as
to let them slip from his grasp. It was the same old story of human
progress; the sacrifice of the individual to the race. For Darwin's loss
was the world's gain, and if he had not limited himself to one line of
effort, and given himself up to that work to the exclusion of everything
else, the world might still be waiting for the _Origin of Species_, and
the revolution in human thought and human life which followed in the
wake of that great book. Carlyle defined genius as an infinite capacity
for taking pains. George Eliot characterized it as an infinite capacity
for receiving discipline. But to make the definition complete, we need
the formulation of Goethe, who identified genius with the power of
concentration: "Who would be great must limit his ambitions; in
concentration is shown the Master."

And so the great men of history, from the very fact of their genius, are
apt not to correspond with what our ideal of greatness demands. Indeed,
our ideal is often more nearly realized in men who fall far short of
genius. When I studied chemistry, the instructor burned a bit of diamond
to prove to us that the diamond was, after all, only carbon in an
"allotropic" form. There seems to be a similar allotropy working in
human nature. Some men seem to have all the constituents of genius, but
they never reach very far above the plane of the commonplace. They are
like the diamond,--except that they are more like the charcoal.

I wish to describe to you a teacher who was not a genius, and yet who
possessed certain qualities that I should abstract and appropriate if I
were to construct in my imagination an ideal teacher. I first met this
man five years ago out in the mountain country. I can recall the
occasion with the most vivid distinctness. It was a sparkling morning,
in middle May. The valley was just beginning to green a little under
the influence of the lengthening days, but on the surrounding mountains
the snow line still hung low. I had just settled down to my morning's
work when word was brought that a visitor wished to see me, and a moment
later he was