Different
countries have been examined and compared with each other, and many of
the differences accounted for; and in Mr Henfrey's volume we have an
acceptable _resume_ of these various researches.
It becomes necessary, first of all, to study the influences--whether
general or special--which affect the distribution of vegetation; to
inquire into those freaks or aberrations of nature which favour in one
place the production of plants that will not grow in another, under
apparently similar circumstances; and why similar plants are found in
places widely separated. Oranges will ripen on one side the Alps, but
not on the other; grapes scarcely come to perfection out of doors in
England, while on the other side of the Channel they ripen by
thousands of acres; and several fruits which fail in our northern
counties, are grown without difficulty in Denmark in the open air.
Investigation soon shewed that temperature alone, mere heat and cold,
was insufficient to account for the phenomena; but that moisture and
dryness, the prevalence of certain winds, the chemical and physical
conditions of soil, and the constitution of the plants themselves,
would have to be considered in a proper inquiry into the subject.
Here we must notice a fact which has proved of essential service in
the study of botanical geography--namely, the discovery 'that there is
some law presiding over the distribution of plants which causes the
appearance of particular species arbitrarily--if we may so say it--in
particular places;' from which, the conclusion has been arrived at,
'that countries have become populated with plants partly by the
spreading of some special kinds from centres within those countries
where they were originally exclusively created; and while these have
spread outward into the neighbouring regions, colonists from like
centres lying in the surrounding countries have invaded and become
intermingled with the indigenous inhabitants.
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