406 
GEOGRAPHCAL DISTRIBUTIOK. 
Chap. XII. 
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 re¬ 
taining 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, pro¬ 
bably accounts for a law which has long been observed, 
and which has lately been admirably discussed by Alph. 
de Candolle in regard to plants, namely, that the lower 
any group of organisms is, the more widely it is apt to 
range. 
The relations just discussed,—namely, low and slowly- 
changing organisms ranging more widely than the 
high,—some of the species of widely-ranging genera 
themselves ranging widely,—such facts, as alpine, lacus¬ 
trine, and marsh productions being related (with the 
exceptions before specified) to those on the surrounding 
low lands and dry lands, though these stations are so 
different,—the very close relation of the distinct species 
which inhabit the islets of the same archipelago,—and 
especially the striking relation of the inhabitants of 
each whole archipelago or island to those of the nearest 
mainland,—are, I think, utterly inexplicable on the 
ordinary view of the independent creation of each spe¬ 
cies, but are explicable on the view of colonisation 
from the nearest or readiest source, together with the 
subsequent modification and better adaptation of the 
colonists to their new homes. 
Summary of 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 all 
