. It would be most difficult to give any rational
explanation of the affinities of the blind cave-animals to the other
inhabitants of the two continents on the ordinary view of their independent
creation. That several of the inhabitants of the caves of the Old and New
Worlds should be closely related, we might expect from the well-known
relationship of most of 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, &c., 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. {140} 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