The Explanation of Adaptations. 607 
may be of interest to show that the prevailing view, ac- 
cording to which WALLACE'S form of the theory of selec- 
tion is the only one which will account for adaptations, 
is erroneous. 
The view that all the characters of organisms vary 
in every desired direction, and that the slightest deviation 
may be subjected to the struggle for existence, and can 
be accentuated to, and finally fixed at, the necessary de- 
gree of development, is certainly an extremely convenient 
one. I willingly admit that almost anything can be 
squared with this theory in a very plausible way, and 
that explanations of this kind are very attractive to the 
student ; but this is not science. The contradictions in 
such a system must be satisfactorily explained before it 
is accepted ; and if we attempt to do this, we soon come to 
the conclusion that the hypothesis itself is not in harmony 
with the available evidence. 
The limits of the applicability of the theory of selec- 
tion, as applied to this question, are known to everybody ; 
and without doubt they are extremely wide. How much 
the theory of mutation has to offer in this respect we 
do not know, because no attempt to estimate this has as 
yet been made, but everything points to the conclusion 
that this theory will explain adaptations just as com- 
pletely, or rather just as incompletely, as the present 
view. It will, however, always have the special feature 
of emphasizing the hypothetical parts of the argument, 
rather than of dismissing them into the background. 
At the present time the theory of selection has still 
the larger number of adherents ; but amongst the younger 
investigators a train of thought is developing which, as 
we have seen above, ascribes a greater importance to 
discontinuous changes. For them fluctuating variability 
