The Making of Species 
books which come under the third category have 
the defects of extreme youth. De Vries has 
discovered a new principle, and it is but natural 
that he should exaggerate its importance, and see 
in it more than it contains. But, as time wears 
on, these faults will disappear, and the theory of 
mutations will assume its true form and fall into 
its proper place, which is somewhere between 
the dustbin, to which Wallaceians would relegate 
it, and the exalted pinnacle on to which De 
Vries would elevate it. 
In the present state of our knowledge, books 
of Class IV. are the most useful to the student, 
since they are unbiassed, and contain a judicial 
summing-up of the evidence for and against the 
various evolutionary theories which now occupy 
the field. Their chief defect is that they are 
almost entirely destructive. They shatter the 
faith of the reader, but offer nothing in place 
of that which they have destroyed. T. H. 
Morgan’s Evolutzon and Adaptation, however, 
contains much constructive matter, and so is the 
most valuable work of this class in existence. 
Zoological science stands in urgent need of 
constructive books on evolution— books with 
leanings towards neither Wallaceism, nor La- 
marckism, nor De Vriesism ; books which shall 
set forth facts of all kinds, concealing none, 
not even those which do not admit of explana- 
tion in the present state of our knowledge.— 
vi 
