GEOGRAPHICAL DISTRIBUTION 206 



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 geo- 

 logical evidence that within each great class the lower 

 organisms change at a slower rate than the higher; con- 

 sequently 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 pro- 

 ductions 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 colonization from the nearest or readiest 

 source, together with the subsequent adaptation of the 

 colonists to their new homes. 



