364 ON THE ORIGIN OF SPECIES 



but, if the variation had been a little greater, the two 

 varieties would have been ranked as distinct 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- 

 scended 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 favour- 

 able for the conversion of its offspring, firstly into new 

 varieties and ultimately into new species. 



In considering the wide distribution of certain 

 genera, we should bear in mind that some are ex- 

 tremely 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 cli- 

 matal and geographical changes and for accidents of 

 transport ; and consequently for the migration of some 

 of the species into all quarters of the world, where 

 they may have become slightly modified in relation to 

 their new conditions. There is, also, some reason to 

 believe from geological evidence that organisms low in 

 the scale within each great class, generally change at a 

 slower rate than the higher forms ; and consequently 

 the lower forms will have had a better chance of ranging 

 widely and of still retaining the same specific character. 

 This fact, together with the seeds and eggs of many low 

 forms being very minute and better fitted for distant 

 transportation, probably accounts for a law which has 

 long been observed, and which has lately been admirably 



