PREFACE 
OST-DARWINIAN books on evolution 
fall naturally into four classes. I. Those 
which preach Wallaceism, as, for ex- 
ample, Wallace’s Darwzxzsm, Poulton’s 
Essays on Evolution, and the voluminous works 
of Weismann. II. Those advocating Lamarckism. 
Cope’s Factors of Evolutzon and the writings of 
Haeckel belong to this class. III. The writings 
of De Vries, forming a group by themselves. 
They advocate the theory that species spring 
suddenly into being; that new species arise by 
mutations from pre-existing species. IV. The 
large number of books of a more judicial nature, 
books written by men who decline to subscribe 
to any of the above three creeds. Excellent 
examples of such works are Kellog’s Darwinism 
To-Day, Lock’s Recent Progress in the Study of 
Variation, Heredity, and Evolution, and T. H. 
Morgan’s Evolutzon and Adaptation. 
All four classes are characterised by defects. 
Books of the two first classes exhibit the 
faults of ardent partisanship. They formulate 
creeds, and, as Huxley truly remarked, ‘“ Science 
commits suicide when it adopts a creed.” The 
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