examination of a positive fact, which is more general than is
supposed, and which has not received the attention it deserves,
doubtless because, very often, it is quite difficult to discover.
This fact consists in the influence which circumstances exert on the
different organisms subjected to them.

"In truth, for a long time there has been noticed the influence of
different states of our organization on our character, our
propensities (_penchants_), our actions, and even our ideas; but it
seems to me that no one has yet recognized that of our actions and
of our habits on our organization itself. Now, as these actions and
these habits entirely depend on the circumstances in which we
habitually find ourselves, I shall try to show how great is the
influence which these circumstances exercise on the general form, on
the condition of the parts, and even on the organization of living
bodies. It is therefore this very positive fact which is to be the
subject of this chapter.

"If we have not had numerous occasions to plainly recognize the
effects of this influence on certain organisms which we have
transported under entirely new and different circumstances, and if
we had not seen these effects and the changes resulting from them
produced, in a way, under our very eyes, the important fact in
question would have always remained unknown.

"The influence of circumstances is really continuously and
everywhere active on living beings, but what renders it difficult
for us to appreciate this influence is that its effects only become
sensible or recognizable (especially in the animals) at the end of a
long period.

"Before stating and examining the proofs of this fact, which
deserves our attention, and which is very important for a zooelogical
philosophy, let us resume the thread of the considerations we had
begun to discuss.

"In the preceding paragraph we have seen that it is now an
incontrovertible fact that, in considering the animal scale in a
sense the inverse of that of nature, we find that there exists in
the groups composing this scale a continuous but irregular
modification (_degradation_) in the organization of animals which
they comprise, an increasing simplification in the organization of
these organisms; finally, a proportionate diminution in the number
of faculties of these beings.

"This fact once recognized may throw the greatest lig