358 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 



It will, I believe, be found universally true, that wnerever in two 

 regions, let them be ever so distant, many closely allied or repre- 

 sentative species occur, there will likewise be found some identical 

 species ; and wherever many closely-allied species occur, there will 

 be found many forms which some naturalists rank as distinct 

 species, and others as mere varieties ; these doubtful forms showing 

 us the steps in the progress of modification. 



The relation between the power and extent of migration in 

 certain species, either at the present or at some former 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 this rule is generally true, though difficult of proof. 

 Amongst mammals, we see it strikingly displayed in Bats, and 

 in a lesser degree in the FelidEe and Canida;. 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 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 important 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 do find, that some at least of 

 the species range very widely. 



We should bear in mind that many genera in all classes are of 

 ancient origin, and the species in this case will have had ample 

 time for dispersal and subsequent modification. There is also 

 reason to believe from geological evidence, that within each great 



