ffspring, firstly into new
varieties and ultimately into new species.

In considering the wide distribution of certain genera, we should bear
in mind that some are extremely ancient, and must have branched off from
a common parent at a remote epoch; so that in such cases there will
have been ample time for great climatal and geographical changes and for
accidents of transport; and consequently for the migration of some of
the species into all 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 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 and 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 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