CiiAr. XII. ISLANDS TO THOSE OF THE MAIN-LAND. 3^7 



range over tlic world, many of the species have very wide 

 rana^es. I can hardly doubt that this rule is generally true, 

 though it would be difficult to prove it. Among mammals, we 

 see it strikingly displayed in bats, and in a lesser degree in 

 the Felid;e and CanidiV. We see the same rule in the distribu- 

 tion of butterflies and beetles. So it is with most of the in- 

 habitants of fresh water, for many of the genera in the most 

 distinct classes range over the world, and many of the species 

 have enonnous ranges. It is not meant that all^ but that some 

 of the species in the genera which range very widely, have 

 themselves very wide ranges. 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 modilicatiou has 

 gone; for instance, two varieties of the same species inhabit 

 America and Europe, and thus the species has an immense 

 rjtnge ; but, if variation were to be carried a little further, the 

 two varieties would be ranked as distinct species, and the 

 range Avould 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 powerfullj'-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 ^'ictorious in distant 

 lands in the struggle for life with foreign associates. But, ac- 

 cording to the vi(.'w that all the species of the same genus, 

 though now distributed at the most remote points of the worhl, 

 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, in relation to all organic beings, 

 that many genera arc of very ancient origin, and the species in 

 this case will have had ample time for dispersal and subsequent 

 modificati(.)n. There is also reason to believe, fiom geological 

 evidence, that Avithin each great class the lower organisms 

 change at a slower rate than the higher; consequently they 

 will liave had a better chance of ranging widely and of still re- 

 taining the same sjjeeific character. This fact, together with 

 the seeds and eggs of almost all lowly-organized forms l)eing 

 very minute and better fitted for distant transportal, probably 

 accounts for a law which has long been observed, and which 

 has lately been discussed by Alpli. de CandoUe in regard to 

 plants, namely, that the lower any group of organisms stands, 

 the more widely it ranges. 



