CASES OF ADAPTATION 167 



difficulty whatever in accepting the " origin of species " from 

 other species as a demonstrated fact ; and this alone was the 

 hitherto insoluble problem Avhich Darwin first succeeded in 

 solving. It is only in the extension of the process to isolated 

 groups such as the whales, the elephants, the serpents, or the 

 mammalia; or by enquiring how special organs, such as horns, 

 teeth, ears, or eyes, could have begun their process of develop- 

 ment, that difficulties 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 inter- 

 mediate stages either through the investigations of embryolo- 

 gists, or of palaeontologists, so that many of the greatest diffi- 

 culties of Darwin's early opponents have quite disappeared. 

 Some of these recent explanations have been referred to al- 

 ready, and many others are briefly described in my Darwin- 

 ism. 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 sugges- 

 tions beyond the scope of Dar^vin's work, and which, I think, 

 have not been sufficiently considered by later writers on evolu- 

 tion. 



