ssed 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 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 of
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 adaptation of the colonists to their new homes.

SUMMARY OF THE 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 changes of climate
and of the level of the land, which have certainly occurred within the
recent period, and of other changes which have probably occurred--if we
remember how ignorant we are with respect to the many curious means of
occasional transport--if we bear in mind, and this is a very important
consideration, how often a species may have ranged continuously over a
wide area, and then have become extinct in the intermediate tracts--the
difficulty is not insuperable in believing that all the individuals of
the same species, wherever found, are descended from common parents.
And we are led to this conclusion, which has been arrived at by many
naturalists under the designation of single centres of creation, by
various general considerations, more especially from the importance
of barriers of all kinds, and from the analogical distribution of
subgenera, genera, and families.

With respect to distinct species belonging to the same genus, which
on our theory have spread from one parent-source; if we make the same
allowances as before for our ignorance, and remember that some forms of
life have changed very slowly, enormous periods of time having been thus
granted for their migration, the difficulties are far from insuperable;
though in this case, as in that of the individuals of the same species,
they are often great.

As exemplifying the effects of climatical changes on distribution, I
have attempted