ISLANDS TO THOSE OF THE MAINLAND. 397 



species has an immense range ; but, if variation were to be 

 carried 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 

 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 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 progenitor, we ought to find, and I believe as a gen- 

 eral 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 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 transportal, 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. 



The relations just discussed — namely, lower organisms 

 ranging more widely than the higher — some of the species 

 of widely ranging genera themselves ranging widely — such 

 facts, as alpine, lacustrine, and marsh productions being gen- 

 erally related to those which live on the surrounding low 

 lands and dry lands — the striking relationship between the 

 inhabitants of islands and those of the nearest mainland — 

 the still closer relationship of the distinct inhabitants of the 

 islauds in the same archipelago — are inexplicable on the 

 ordinary view of the independent creation of each species, 

 but are explicable if we admit colonization from the nearest 

 or readiest source, together with the subsequent ^da/ptation. 

 of the colonists to their new homes, 



