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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