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



other and more general way. Mr. Gould remarked 

 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 this rule is gen- 

 erally true, though difficult of proof. Amongst mam- 

 mals, we see it strikingly displayed in Bats, and in a 

 lesser degree in the Felidse and Canidae. We 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 

 epecies 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 modi- 

 fication 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 which 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 im- 

 portant power of being victorious in distant lands 

 in the struggle for life with foreign 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 rule we 

 38 



