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principle of a plurality of causes is evidently at work, are
we to be content with the assertion which is only a fraction
of the truth, if it be a truth at all, that protective colouring
is the result of Natural Selection? Ought we not to wish
to know the truth, the whole truth? We cannot suppose
that Mr. Wallace wished to arrest further enquiry in the
interests of the theory which he espoused. Such a treat
ment of the subject would only be justified if nature was
known to work always on the principle of “ one effect, one
cause.”
There is one other consideration to be borne in mind,
namely, that the supposed law of parsimony, in asserting
that a given cause is sufficient, by itself to explain a given
result without reference to other causes, always assumes
that this cause is a true cause. I have given reasons for
my belief that Natural Selection is not the true cause of
protective colouring. If this is so, the application of the
law of parsimony to this case would lead not merely to
the inculcation of a half truth, but to the preservation of
error. In no aspect of this subject can the argument
be regarded as worthy of the true scientific spirit.
There is but one proof which is worth anything with
respect to a scientific theory of physical nature, and this is
a rigid comparison of its assumptions with the actual
phenomena. In the earlier part of this work I gave
reasons for believing that the theory of Natural Selection
would not bear this test. It is unnecessary to go over this
ground again, except to say this : that if Natural Selection
is not proved to be active now in the transmutation of
species, it deprives us of all right to assume that it has
been active through untold eras in bringing about the
process of Organic Evolution.
And now we come to what seems to me the most
astounding argument which was ever urged in defence of