Chap. XII. OCEANIC ISLANDS. 405 



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 ; but, if the variation had been a little 

 greater, the two varieties would have been ranked as dis- 

 tinct species, and the common range would have been 

 greatly reduced. Still less is it meant, that a species 

 which apparently has 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 im- 

 portant power of being victorious in distant lands in 

 the struggle for life with foreign associates. But 

 on the view of all the species of a genus having de- 

 descended from a single parent, though now distributed 

 to the most remote points of the world, we ought to 

 find, and I believe as a general rule we do find, that 

 some at least of the species range very widely ; for it is 

 necessary that the unmodified parent should range 

 widely, undergoing modification during its diffusion, and 

 should place itself under diverse conditions favourable 

 for the conversion of its offspring, firstly into new varie- 

 ties and ultimately into new species. 



In considering the wide distribution of certain genera, 

 we should bear in mind that some are extremely ancient, 

 and must have branched off from a common parent 

 at a remote epoch ; so that in such cases there will 

 have been ample time for great climatal and geographical 

 changes and for accidents of transport ; and conse- 

 quently for the migration of some of the species into all 



