eir 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 retaining 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, probably 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, lacustrine, 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 species, 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 {407} the changes of climate and of the level of the
land, which have certainly occurred within the recent period, and of other
similar changes which may have occurred within the same period; if we
remember how profoundly ignorant we are with respect to the many and
curious means of occasional transport,--a subject which has hardly ever
been properly experimentised on; if we bear in mind how often a species may
have ranged continuously over a wide area, and then have become extinct in
the intermediate tracts, I think the difficulties in believing that all the
individuals of the same species, wherever located, have descended from the
same parents, are not insuperable. And we are led to this conclusion