"The alteration which the cetaceans have undergone in their limbs, owing to the influence of the medium in which they live and the habits which they have there contracted, manifests itself also in their fore limbs, which, entirely enveloped by the skin, no longer show externally the fingers in which they end; so that they only offer on each side a fin which contains concealed within it the skeleton of a hand. "Assuredly, the cetaceans being mammals, it entered into the plan of their structure to have four limbs like the others, and consequently a pelvis to sustain their hind legs. But here, as elsewhere, that which is lacking in them is the result of atrophy brought about, at the end of a long time, by the want of use of the parts which were useless. "If we consider that in the Phocæ, where the pelvis still exists, this pelvis is impoverished, narrowed, and with no projections on the hips, we see that the lessened (_médiocre_) use of the hind feet of these animals must be the cause, and that if this use should entirely cease, the hind limbs and even the pelvis would in the end disappear. "The considerations which I have just presented may doubtless appear as simple conjectures, because it is possible to establish them only on direct and positive proofs. But if we pay any attention to the observations which I have stated in this work, and if then we examine carefully the animals which I have mentioned, as also the result of their habits and their surroundings, we shall find that these conjectures will acquire, after this examination, an eminent probability. "The following _tableau_[191] will facilitate the comprehension of what I have just stated. It will be seen that, in my opinion, the animal scale begins at least by two special branches, and that in the course of its extent some branchlets (_rameaux_) would seem to terminate in certain places. "This series of animals beginning with two branches where are situated the most imperfect, the first of these branches received their existence only by direct or spontaneous generation. "A strong reason prevents our knowing the changes successively brought about which have produced the condition in which we observe them; it is because we are never witnesses of these changes. Thus we see the work when done, but never watching them during the process, we are naturally led to believe that things have always been as we see them, and not as they have progressively been brought about. "Among the changes which nature everywhere incessantly produces in her _ensemble_, and her laws remain always the same, such of these changes as, to bring about, do not need much more time than the duration of human life, are easily understood by the man who observes them; but he cannot perceive those which are accomplished at the end of a considerable time.