"We know that the Infusoria, the polyps, and the Radiata only live in the water; that the worms even only live some in the water and others in very damp places. "Moreover, regarding the worms, which seem to form an initial branch of the animal scale, since it is evident that the Infusoria form another branch, we may suppose that among those of them which are wholly aquatic--namely, which do not live in the bodies of other animals, such as the Gordius and many others still unknown--there are doubtless a great many different aquatic forms; and that among these aquatic worms, 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 Scolopendræ 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.