their other productions. Far from feeling any surprise that some of the
cave-animals should be very anomalous, as Agassiz has remarked in
regard to the blind fish, the Amblyopsis, and as is the case with the
blind Proteus with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the inhabitants of these
dark abodes will probably have been exposed.
_Acclimatisation_. Habit is hereditary with plants, as in the period of
flowering, in the amount of rain requisite for seeds to germinate, in
the time of sleep, etc., and this leads me to say a few words on
acclimatisation. As it is extremely common for species of the same
genus to inhabit very hot and very cold countries, and as I believe
that all the species of the same genus have descended from a single
parent, if this view be correct, acclimatisation must be readily
effected during long-continued descent. It is notorious that each
species is adapted to the climate of its own home: species from an
arctic or even from a temperate region cannot endure a tropical
climate, or conversely. So again, many succulent plants cannot endure a
damp climate. But the degree of adaptation of species to the climates
under which they live is often overrated.
We may infer this from our frequent inability to predict whether or not
an imported plant will endure our climate, and from the number of
plants and animals brought from warmer countries which here enjoy good
health. We have reason to believe that species in a state of nature are
limited in their ranges by the competition of other organic beings
quite as much as, or more than, by adaptation to particular climates.
But whether or not the adaptation be generally very close, we have
evidence, in the case of some few plants, of their becoming, to a
certain extent, naturally habituated to different temperatures, or
becoming acclimatised: thus the pines and rhododendrons, raised from
seed collected by Dr. Hooker from trees growing at different heights on
the Himalaya, were found in this country to possess different
constitutional powers of resisting cold. Mr. Thwaites informs me that
he has observed similar facts in Ceylon, and analogous observations
have been made by Mr. H. C. Watson on European species of plants
brought from the Azores to England. In regard to animals, several
authentic cases could be given of species within historical times
having largely extended their range from warmer to cooler latitudes,
and conversely; but we do not positively know that these animals were
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