s, those which afterwards habitually expose
themselves to the air have probably produced amphibious insects,
such as the mosquitoes, the ephemeras, etc., etc., which have
successively given origin to all the insects which live solely in
the air. But several races of these having changed their habits by
the force of circumstances, and having formed habits of a life
solitary, retired, or hidden, have given rise to the arachnides,
almost all of which also live in the air.

"Finally, those of the arachnides which have frequented the water,
which have consequently become progressively habituated to live in
it, and which finally cease to expose themselves to the air--this
indicates the relations which, connecting the Scolopendrae to Julus,
this to the Oniscus, and the last to Asellus, shrimps, etc., have
caused the existence of all the Crustacea.

"The other aquatic worms which are never exposed to the air,
multiplying and diversifying their races with time, and gradually
making progress in the complication of their structure, have caused
the formation of the Annelida, Cirripedia, and molluscs, which
together form an uninterrupted portion of the animal scale.

"In spite of the considerable hiatus which we observe between the
known molluscs and the fishes, the molluscs, whose origin I have
just indicated, have, by the intermediation of those yet remaining
unknown, given origin to the fishes, as it is evident that the
latter have given rise to the reptiles.

"In continuing to consult the probabilities on the origin of
different animals, we cannot doubt but that the reptiles, by two
distinct branches which circumstances have brought about, have given
rise on one side to the formation of birds, and on the other to
that of amphibious mammals, which have given in their turn origin to
all the other mammals.[190]

"Indeed, the fishes having caused the formation of Batrachia, and
these of the Ophidian reptiles, both having only one auricle in the
heart, nature has easily come to give a heart with a double auricle
to other reptiles which constitute two special branches; finally,
she has easily arrived at the end of forming, in the animals which
had originated from each of these branches, a heart with two
ventricles.

"Thus, among the reptiles whose heart has a double auricle, on the
one side, the Chelonians seem to have given origin to the birds; if,