constitute the final aim of scientific
research, obviously the advancement of such research can be attained
only by the framing of hypotheses. And to frame hypotheses is to
speculate.

Therefore, the difference between science and speculation is not a
difference of spirit; nor, thus far, is it a difference of method. The
only difference between them is in the subsequent process of verifying
hypotheses. For while speculation, in its purest form, is satisfied to
test her explanations only by the degree in which they accord with our
subjective ideas of probability--or with the "Illative Sense" of
Cardinal Newman,--science is not satisfied to rest in any explanation as
final until it shall have been fully verified by an appeal to objective
proof. This distinction is now so well and so generally appreciated that
I need not dwell upon it. Nor need I wait to go into any details with
regard to the so-called canons of verification. My only object is to
make perfectly clear, first, that in order to have any question to put
to the test of objective verification, science must already have so far
employed the method of speculation as to have framed a question to be
tested; and, secondly, that the point where science parts company with
speculation is the point where this testing process begins.

Now, if these things are so, there can be no doubt that Darwin was
following the truest method of inductive research in allowing any amount
of latitude to his speculative thought in the direction of scientific
theorizing. For it follows from the above distinctions that the danger
of speculation does not reside in the width of its range, or even in the
impetuosity of its vehemence. Indeed, the wider its reach, and the
greater its energy, the better will it be for the interests of science.
The only danger of speculation consists in its momentum being apt to
carry away the mind from the more laborious work of adequate
verification; and therefore a true scientific judgment consists in
giving a free rein to speculation on the one hand, while holding ready
the break of verification with the other. Now, it is just because Darwin
did both these things with so admirable a judgment, that he gave the
world of natural history so good a lesson as to the most effectual way
of driving the chariot of science.

This lesson we have now all more or less learnt to profit by. Yet no
other naturalist has proved himself so proficient in holding the balance
true.