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to undergo the process of acclimatisation. This is often
very far from easy when man takes all the care he can to
guard the exotics which he imports. It would, of course,
be still more difficult in the case of the organism left to
fight its own battle unaided. One special point may be
mentioned here. The protection which an animal or plant
possesses in one locality is apt to disappear very shortly
when it is removed to a different sphere. The desert
plant loses its spines,—
“ The hemlock is said not to yield conicine in Scotland. The root
of the Aconitum 7iaft elites becomes innocuous in frigid climates. The
medicinal properties of the Digitalis are easily affected by culture.
The rhubarb flourishes in England, but does not produce the
medicinal substance which makes the plant so valuable in Chinese
Tartarv. In the South of France the Pistacia lentiscus yields no
mastic. Hemp fails to produce in England that resinous matter
which is so largely used in India as an intoxicating drug.”—(The
Variation, vol. ii., ftp. 274.-5.)
The argument for Organic Evolution deduced from the
geographical distribution of animals and plants, implies
that species undergo modification to adapt them to the
new conditions into which they are thrown by their volun
tary and involuntary emigration. It is an argument for
the transmutation, as opposed to the fixity, of species.
The process is a perfectly intelligible one. The district
overcrowded by the individuals of a given species sends
forth its emigrants, who find a modus vivendi in the new
sphere. Change of structure ensues, sometimes useful and
sometimes unuseful. The process may have its difficulties,
which are fatal to individuals but it is successful so far as
the race of emigrants are concerned. The species survives
in a modified form, because, on the whole, the struggle for
existence does not press too heavily upon it. But if you
assert that this transmutation has taken place by means of
Natural Selection, and you introduce all the conditions
