"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 _dégradation_ of their organization. But this modification (_dégradation_) 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 the lamprey and fishes to the mammals is not a regularly gradated one, and accounts for this "because the work of nature has been often changed, hindered, and diverted in direction by the influences which singularly different, even contrasted, circumstances have exercised on the animals which are there found exposed in the course of a long series of their renewed generations." Lamarck thus accounts for the production of the radial symmetry of the medusæ and echinoderms, his _Radiaires_. At the present day this symmetry is attributed perhaps more correctly to their more or less fixed mode of life. "It is without doubt by the result of this means which nature employs, at first with a feeble energy with _polyps_, and then with greater developments in the _Radiata_, that the radial form has been acquired; because the subtile ambient fluids, penetrating by the alimentary canal, and being expansive, have been able, by an incessantly renewed repulsion from the centre towards every point of the circumference, to give rise to this radiated arrangement of parts. "It is by this cause that, in the Radiata, the intestinal canal, although still very imperfect, since more often it has only a single opening, is yet complicated with numerous radiating vasculiform, often ramified, appendages.