204 THE ORIGIN OF SPECIES 



lormer period, and the existence at remote points of the 

 world of closely-allied species, is shown in another 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 tbis rule is generally true, 

 though difficult of proof. Among mammals, we see it 

 strikingly displayed in Bats, and in a lesser degree in 

 the Felidae 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 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 in- 

 stance, 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 

 important power of being victorious in distant lands in 

 the struggle for life with foreign associates. But accord- 

 ing to the view that all the species of a genus, though 

 distributed to the most remote points of the world, are 



I 



