SKETCH OF THE "ORIGIN." 47 



producing variation, and to the inheritance of acquired 

 habits than in the ' Origin/ " while Professor Newton 

 pointed out that the remarks on the migration of 

 birds anticipate the views of later writers."^ 



The explanation of divergence of species during 

 modification (divergence of character) had not then 

 occurred to him, and he tells us in the " Auto- 

 biography : — 



" I can remember the very spot in the road, whilst in my 

 carriage, when to my joy the solution occurred to me ; and 

 this was long after I had come to Down. The solution, as I 

 believe, is that the modified offspring of all dominant and 

 increasing forms tend to become adapted to many and highly 

 diversified places in the economy of nature." 



A good example of this tendency is seen in 

 the relations of three great vertebrate classes — 

 mammals, birds, and fishes — to the environments 

 for which they are respectively fitted: earth, air, 

 and water. Competition is most severe between 

 forms most nearly alike, and hence some measure 

 of relief from competition is afforded when certain 

 members of each of these classes enter the domain 

 of one of the others. Hence, we observe that although 

 mammals as a whole are terrestrial, a small minority 

 are aerial and aquatic ; although birds are aerial, a 

 minority are terrestrial and aquatic ; although fishes 

 are aquatic, a minority tend to be, at any rate largely, 

 terrestrial and aerial. 



* Professor H. F. Osborn has rightly urged that this essay 

 should be published (" From the Greeks to Darwin," 1894, 

 p. 235). 



