CHAP. XIII.J ISLANDS TO THOSE OF THE MAINLAND. 337 



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 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 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, to- 

 gether with that of the seeds and eggs of most lowly organised 

 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 generally 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 dis- 

 tinct inhabitants of the islands in the same archipelago are 

 inexplicable on the ordinary view of the independent creation of 

 each species, but are explicable if we admit colonisation from the 

 nearest or readiest source, together with the subsequent adaptation 

 of the colonists to their new homes. 



Summary of the last and present Chapters. 



In these chapters I have endeavoured to show, that if we make 

 due allowance for our ignorance of the full effects of changes of 

 climate and of the level of the land, which have certainly occurred 

 within the recent period, and of other changes which have probably 

 occurred, if we remember how ignorant we are with respect to 

 the many curious means of occasional transport, if we bear in 

 mind, and this is a very important consideration, how often a 

 species may have ranged continuously over a wide area, and then 

 have become extinct in the intermediate tracts, the difficulty ir 



