nute and very often insufficient.

"From this state of things well established we see what are the
causes which have given rise to them; we see whether nature
possesses the means for this, and if observation has been able to
give us our explanation of it.

"A great many facts teach us that gradually as the individuals of
one of our species change their situation, climate, mode of life, or
habits, they thus receive influences which gradually change the
consistence and the proportions of their parts, their form, their
faculties, even their organization; so that all of them participate
eventually in the changes which they have undergone.

"In the same climate, very different situations and exposures at
first cause simple variations in the individuals which are found
exposed there; but, as time goes on, the continual differences of
situation of individuals of which I have spoken, which live and
successively reproduce in the same circumstances, give rise among
them to differences which are, in some degree, essential to their
being, in such a way that at the end of many successive generations
these individuals, which originally belonged to another species, are
at the end transformed into a new species, distinct from the other.

"For example, if the seeds of a grass, or of every other plant
natural to a humid field, should be transplanted, by an accident, at
first to the slope of a neighboring hill, where the soil, although
more elevated, would yet be quite cool (_frais_) so as to allow the
plant to live, and then after having lived there, and passed through
many generations there, it should gradually reach the poor and
almost arid soil of a mountain side--if the plant should thrive and
live there and perpetuate itself during a series of generations, it
would then be so changed that the botanists who should find it there
would describe it as a separate species.

"The same thing happens to animals which circumstances have forced
to change their climate, manner of living, and habits; but for these
the influences of the causes which I have just cited need still more
time than in the case of plants to produce the notable changes in
the individuals, though in the long run, however, they always
succeed in bringing them about.

"The idea of defining under the word _species_ a collection of
similar individuals which perpetuate the same by generation, and