d.

"In the same climate very different situations and exposures cause
simple variations in the individuals occurring there; but, after the
lapse of time, the continual differences of situation of the
individuals of which I speak, which live and successively reproduce
under the same circumstances, produce differences in them which
become, in some degree, essential to their existence, so that at the
end of many successive generations these individuals, which
originally belonged to another species, became finally transformed
into a new species distinct from the other.

"For example, should the seeds of a grass or of any other plant
natural to a moist field be carried by any means at first to the
slope of a neighboring hill, where the soil, although more elevated,
will yet be sufficiently moist to allow the plant to live there, and
if it results, after having lived there and having passed through
several generations, that it gradually reaches the dry and almost
arid soil of a mountain side; if the plant succeeds in living there,
and perpetuates itself there during a series of generations, it will
then be so changed that any botanists who should find it there would
make a distinct species of it.

"The same thing happens in the case of animals which circumstances
have forced to change in climate, mode of life, and habits; but in
their case the influences of the causes which I have just cited need
still more time than the plants to bring about notable changes in
the individuals.

"The idea of embracing, under the name of _species_, a collection of
like individuals which are perpetuated by generation, and which have
remained the same as long as nature has endured, implies the
necessity that the individuals of one and the same species should
not cross with individuals of a different species.

"Unfortunately observation has proved, and still proves every day,
that this consideration is unfounded; for hybrids, very common among
plants, and the pairings which we often observe between the
individuals of very different _species_ of animals, have led us to
see that the limits between these supposed constant species are not
so fixed as has been imagined.

"In truth, nothing often results from these singular unions,
especially if they are very ill-assorted, and then the individuals
which do result from them are usually infertile; but also, when the
di