ducing that flash of insight which led us
immediately to the simple but universal law of the "survival of
the fittest," as the long-sought _effective_ cause of the
continuous modification and adaptations of living things.

It is an unimportant detail that Darwin read this book two years
_after_ his return from his voyage, while I read it _before_ I
went abroad, and it was a sudden recollection of its teachings
that caused the solution to flash upon me. I attach much
importance, however, to the large amount of solitude we both
enjoyed during our travels, which, at the most impressionable
period of our lives, gave us ample time for reflection on the
phenomena we were daily observing.

This view, of the combination of certain mental faculties and
external conditions that led Darwin and myself to an identical
conception, also serves to explain why none of our precursors or
contemporaries hit upon what is really so very simple a solution
of the great problem. Such evolutionists as Robert Chambers,
Herbert Spencer, and Huxley, though of great intellect, wide
knowledge, and immense power of work, had none of them the special
turn of mind that makes the collector and the species-man; while
they all--as well as the equally great thinker on similar lines,
Sir Charles Lyell--became in early life immersed in different
lines of research which engaged their chief attention.

Neither did the actual precursors of Darwin in the statement of
the principle--Wells, Matthews and Prichard--possess any adequate
knowledge of the class of facts above referred to, or sufficient
antecedent interest in the problem itself, which were both needed
in order to perceive the application of the principle to the mode
of development of the varied forms of life.

And now, to recur to my own position, I may be allowed to make a
final remark. I have long since come to see that no one deserves
either praise or blame for the _ideas_ that come to him, but only
for the actions resulting therefrom. Ideas and beliefs are
certainly not voluntary acts. They come to us--we hardly know
_how_ or _whence_, and once they have got possession of us we
cannot reject or change them at will. It is for the common good
that the promulgation of ideas should be free--uninfluenced either
by praise or blame, reward or punishment.