INHABITANTS OF ISLANDS 445 



riod, 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 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 instance, two varieties of the 

 same species inhabit America and Europe, and thus the spe- 

 cies has an immense range; but, if variation were to be car- 

 ried 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 cer- 

 tain 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 pro- 

 genitor, 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 class the lower organisms change at a slower rate 

 than the higher; consequently they will have had a better 

 chance of ranging widely and of still retaining the same spe- 

 cific character. This fact, together with that of the seeds 



