GEOGRAPHICAL DISTRIBUTION 363 



lakes and marshes, excepting in so far as great facility 

 of transport has given the same general forms to the 

 whole world. We see this same principle in the blind 

 animals inhabiting the caves of America and of Europe. 

 Other analogous facts could be given. And it will, I 

 believe, be universally found to be true, that wherever 

 in two regions, let them be ever so distant, many 

 closely-allied or representative species occur, there will 

 likewise be found some identical species, showing, in 

 accordance with the foregoing view, that at some 

 former period there has been intercommunication or 

 migration between the two regions. And wherever 

 many closely-allied species occur, there will be found 

 many forms which some naturalists rank as distinct 

 species, and some as varieties ; these doubtful forms 

 showing us the steps in the process of modification. 



This relation between the power and extent of 

 migration of a species, either at the present time or at 

 some former period under different physical conditions, 

 and the existence at remote points of the world of 

 other species allied to it, 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 this rule is generally true, 

 though it would be difficult to prove it. Amongst 

 mammals, we see it strikingly displayed in Bats, and 

 in a lesser degree in the Felidae and CanidaB. We see 

 it, if we compare the distribution of butterflies and 

 beetles. So it is with most fresh-water productions, in 

 which so many genera range over the world, and many 

 individual species have enormous ranges. It is not 

 meant that in world - ranging genera all the species 

 have a wide range, or even that they have on an 

 average a wide range ; but only that some of the species 

 range very widely ; for the facility with which widely- 

 ranging species vary and give rise to new forms will 

 largely determine their average range. For instance, 

 two varieties of the same species inhabit America and 

 Europe, and the species thus has an immense range ; 



