ISLANDS TO THOSE OF THE MAINLAND. 423 



I can hardly doubt that this rule is generally true, though 

 difficult of proof. Among mammals, we see it strikingly 

 displayed in bats, and in a lesser degree in the Felidai and 

 Canidae. We see the same rule in the distribution of butter- 

 flies 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 modifica- 

 tion 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 dis- 

 tinct 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 dis- 

 tributed 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 tlie species in this case will 

 have had ample time for dispersal and subsequent modifi- 

 cation. 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 specific character. This fact, together 

 with that of the seeds and eggs of most lowly organized 

 forms being very minute and better fitted for distant trans- 

 portal, probably accounts for a law which has long been 

 observed, and which has lately been discussed by Alph. de 

 Candolle in regard to plants, namely, that the lower any 

 group of organisms stands the more widely it ranges. 



