vin CASES OF ADAPTATION 155 



appear, many of which seem, to some biologists, to be 

 insuperable. But many of these difficult problems have 

 been solved by more complete knowledge ; while others 

 have been rendered easy by the discovery of intermediate 

 stages either through the investigations of embryologists, or 

 of palaeontologists, so that many of the greatest difficulties 

 of Darwin's early opponents have quite disappeared. Some 

 of these recent explanations have been referred to already, 

 and many others are briefly described in my Darwinism. 

 In that work also I have given so many illustrations of the 

 way in which natural selection has worked, that it will be 

 needless for me to go into further details here. I will, 

 therefore, now proceed to an exposition of some problems 

 of a more general nature, which involve difficulties and 

 suggestions beyond the scope of Darwin's work, and which, 

 I think, have not been sufficiently considered by later writers 

 on evolution. 



