passages of this chapter which bear on his theory of
descent. Speaking of the different kinds of aquatic surroundings he
remarks:

"In the first place it should be observed that in the waters
themselves she [Nature] presents considerably diversified
circumstances; the fresh waters, marine waters, calm or stagnant
waters, running waters or streams, the waters of warm climates,
those of cold regions, finally those which are shallow and those
which are very deep, offer many special circumstances, each of which
acts differently on the animals living in them. Now, in a degree
equal to the make-up of the organization, the races of animals which
are exposed to either of these circumstances have been submitted to
special influences and have been diversified by them."

He then, after referring to the general degradation of the Batrachians,
touches upon the atrophy of legs which has taken place in the snakes:

"If we should consider as a result of _degradation_ the loss of legs
seen in the snakes, the _Ophidia_ should be regarded as constituting
the lowest order of reptiles; but it would be an error to admit this
consideration. Indeed, the serpents being animals which, in order to
hide themselves, have adopted the habit of gliding directly along
the ground, their body has lengthened very considerably and
disproportionately to its thickness. Now, elongated legs proving
disadvantageous to their necessity of gliding and hiding, very short
legs, being only four in number, since they are vertebrate animals,
would be incapable of moving their bodies. Thus the habits of these
animals have been the cause of the disappearance of their legs, and
yet the _batrachians_, which have them, offer a more degraded
organization, and are nearer the fishes" (p. 155).

Referring on the next page to the fishes, he remarks:--

"Without doubt their general form, their lack of a constriction
between the head and the body to form a neck, and the different fins
which support them in place of legs, are the results of the
influence of the dense medium which they inhabit, and not that of
the _degradation_ of their organization. But this modification
(_degradation_) is not less real and very great, as we can convince
ourselves by examining their internal organs; it is such as to
compel us to assign to the fishes a rank lower than that of the
reptiles."

He then states that the series from th