196 RELATIONS OF THE INHABITANTS OF [Chap. XIII. 



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 sub- 

 sequent modification. Tliere 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 organised forms 

 being very minute and better fitted for distant trans- 

 portal, probably accounts for a law which has long 

 been observed, and which has lately been discussed by 

 Alph. de CandoUe in regard to plants, namely, that 

 the lower any group of organisms stands the more 

 ■ widely it ranges. 



The relations just discussed, — namely, lower organ- 

 isms 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 distinct 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 adap- 

 tation of the colonists to their new homes. 



