Chap. XIII.] ISLANDS TO THOSE OF THE MAINLAND. 195 



in another and more general way. Mr. Gould re- 

 marked to me long ago, that in those genera of birds 

 which range over the world, many of the species have 

 very wide ranges. I can hardly doubt that tliis rule 

 is generally true, though difficult of proof. Amongst 

 mammals, we see it strikingly displayed in Bats, and 

 in a lesser degree in the Felidse and Canidte. "VVe see 

 the same rule in the distribution of butterflies and 

 beetles. So it is with most of the inhabitants of fresh 

 water, for many of the genera in the most distinct 

 classes range over the world, and many of the species 

 have enormous ranges. It is not meant that all, but 

 that some of the species have very wide ranges in the 

 genera which range very widely. Nor is it meant that 

 the species in such genera have on an average a very 

 wide range ; for this will largely depend on how far 

 the process of modification has gone ; for instance, two 

 varieties of the same species inhabit America and 

 Europe, and thus the species has an immense range ; 

 but, if variation were to be carried a little further, 

 the two varieties would be ranked as distinct species, 

 and their range would be greatly reduced. Still less is 

 it meant, that species wliich have the capacity of 

 crossing barriers and ranging widely, as in the case 

 of certain powerfully-winged birds, will necessarily 

 range widely ; for we should never forget that to range 

 widely implies not only the power of crossing barriers, 

 but the more important pov/er of being victorious in 

 distant lands in the struggle for life v/ith foreigu 

 associates. But according to the view that all the 

 species of a genus, though distributed to the most 

 remote points of the world, are descended from a single 

 progenitor, we ought to find, and I believe as a general 



