He then turns to the flying mammals, such as the flying squirrel (Sciurus volans, ærobates, petaurista, sagitta, and volucella), and then explains the origin of their adaptation for flying leaps.
“These animals, more modern than the seals, having the habit of extending their limbs while leaping to form a sort of parachute, can only make a very prolonged leap when they glide down from a tree or spring only a short distance from one tree to another. Now, by frequent repetitions of such leaps, in the individuals of these races the skin of their sides is expanded on each side into a loose membrane, which connects the hind and fore legs, and which, enclosing a volume of air, prevents their sudden falling. These animals are, moreover, without membranes between the fingers and toes.
“The Galeopithecus (Lemur volans), undoubtedly a more ancient form but with the same habits as the flying squirrel (Pteromys Geoff.), has the skin of the flancs more ample, still more developed, connecting not only the hinder with the fore legs, but in addition the fingers and the tail with the hind feet. Moreover, they leap much farther than the flying squirrels, and even make a sort of flight.[189]
“Finally, the different bats are probably mammals still older than the Galeopithecus, in the habit of extending their membrane and even their fingers to encompass a greater volume of air, so as to sustain their bodies when they fly out into the air.
“By these habits, for so long a period contracted and preserved, the bats have obtained not only lateral membranes, but also an extraordinary elongation of the fingers of their fore feet (with the exception of the thumb), between which are these very ample membranes uniting them; so that these membranes of the hands become continuous with those of the flanks, and with those which connect the tail with the two hind feet, forming in these animals great membranous wings with which they fly perfectly, as everybody knows.
“Such is then the power of habits, which have a singular influence on the conformation of parts, and which give to the animals which have for a long time contracted certain of them, faculties not found in other animals.
“As regards the amphibious animals of which I have often spoken, it gives me pleasure to communicate to my readers the following reflections which have arisen from an examination of all the objects which I have taken into consideration in my studies, and seen more and more to be confirmed.
“I do not doubt but that the mammals have in reality originated from them, and that they are the veritable cradle (berceau) of the entire animal kingdom.
“Indeed, we see that the least perfect animals (and they are the most numerous) live only in the water; hence it is probable, as I have said (vol. ii., p. 85), that it is only in the water or in very humid places that nature causes and still forms, under favorable conditions, direct or spontaneous generations which have produced the simplest animalcules and those from which have successively been derived all the other animals.
“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.
“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, independently of several relations which we cannot disregard, I should place the head of a tortoise on the neck of certain birds, I should perceive almost no disparity in the general physiognomy of the factitious animal; and on the other side, the saurians, especially the ‘planicaudes,’ such as the crocodiles, seem to have given origin to the amphibious mammals.
“If the branch of the Chelonians has given rise to birds, we can yet presume that the palmipede aquatic birds, especially the brevipennes, such as the penguins and the manchots, have given origin to the monotremes.
“Finally, if the branch of saurians has given rise to the amphibious mammals, it will be most probable that this branch is the source whence all the mammals have taken their origin.
“I therefore believe myself authorized to think that the terrestrial mammals originally descended from those aquatic mammals that we call Amphibia. Because the latter being divided into three branches by the diversity of the habits which, with the lapse of time, they have adopted, some have caused the formation of the Cetacea, others that of the ungulated mammals, and still others that of the unguiculate mammals.
“For example, those of the Amphibia which have preserved the habit of frequenting the shores differ in the manner of taking their food. Some among them accustoming themselves to browse on herbage, such as the morses and lamatines, gradually gave origin to the ungulate mammals, such as the pachyderms, ruminants, etc.; the others, such as the Phocidæ, contracting the habit of feeding on fishes and marine animals, caused the existence of the unguiculate mammals, by means of races which, while becoming differentiated, became entirely terrestrial.
“But those aquatic mammals which would form the habit of never leaving the water, and only rising to breathe at the surface, would probably give origin to the different known cetaceans. Moreover, the ancient and complete habitation of the Cetacea in the ocean has so modified their structure that it is now very difficult to recognize the source whence they have derived their origin.
“Indeed, since the enormous length of time during which these animals have lived in the depths of the sea, never using their hind feet in seizing objects, their disused feet have wholly disappeared, as also their skeleton, and even the pelvis serving as their attachment.
“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.
“If the duration of human life only extended to the length of a second, and if there existed one of our actual clocks mounted and in movement, each individual of our species who should look at the hour-hand of this clock would never see it change its place in the course of his life, although this hand would really not be stationary. The observations of thirty generations would never learn anything very evident as to the displacement of this hand, because its movement, only being that made during half a minute, would be too slight to make an impression; and if observations much more ancient should show that this same hand had really moved, those who should see the statement would not believe it, and would suppose there was some error, each one having always seen the hand on the same point of the dial-plate.
“I leave to my readers all the applications to be made regarding this supposition.
“Nature, that immense totality of different beings and bodies, in every part of which exists an eternal circle of movements and changes regulated by law; totality alone unchangeable, so long as it pleases its Sublime Author to make it exist, should be regarded as a whole constituted by its parts, for a purpose which its Author alone knows, and not exclusively for any one of them.
“Each part necessarily is obliged to change, and to cease to be one in order to constitute another, with interests opposed to those of all; and if it has the power of reasoning it finds this whole imperfect. In reality, however, this whole is perfect, and completely fulfils the end for which it was designed.”
The last work in which Lamarck discussed the theory of descent was in his introduction to the Animaux sans Vertèbres. But here the only changes of importance are his four laws, which we translate, and a somewhat different phylogeny of the animal kingdom.
The four laws differ from the two given in the Philosophie zoologique in his theory (the second law) accounting for the origin of a new organ, the result of a new need.
“First law: Life, by its proper forces, continually tends to increase the volume of every body which possesses it, and to increase the size of its parts, up to a limit which it brings about.
“Second law: The production of a new organ in an animal body results from the supervention of a new want (besoin) which continues to make itself felt, and of a new movement which this want gives rise to and maintains.
“Third law: The development of organs and their power of action are constantly in ratio to the employment of these organs.
“Fourth law: Everything which has been acquired, impressed upon, or changed in the organization of individuals, during the course of their life is preserved by generation and transmitted to the new individuals which have descended from those which have undergone those changes.”
In explaining the second law he says:
“The foundation of this law derives its proof from the third, in which the facts known allow of no doubt; for, if the forces of action of an organ, by their increase, further develop this organ—namely, increase its size and power, as is constantly proved by facts—we may be assured that the forces by which it acts, just originated by a new want felt, would necessarily give birth to the organ adapted to satisfy this new want, if this organ had not before existed.
“In truth, in animals so low as not to be able to feel, it cannot be that we should attribute to a felt want the formation of a new organ, this formation being in such a case the product of a mechanical cause, as that of a new movement produced in a part of the fluids of the animal.
“It is not the same in animals with a more complicated structure, and which are able to feel. They feel wants, and each want felt, exciting their inner feeling, forthwith sets the fluids in motion and forces them towards the point of the body where an action may satisfy the want experienced. Now, if there exists at this point an organ suitable for this action, it is immediately cited to act; and if the organ does not exist, and only the felt want be for instance pressing and continuous, gradually the organ originates, and is developed on account of the continuity and energy of its employment.
“If I had not been convinced: 1, that the thought alone of an action which strongly interests it suffices to arouse the inner feeling of an individual; 2, that a felt want can itself arouse the feeling in question; 3, that every emotion of inner feeling, resulting from a want which is aroused, directs at the same instant a mass of nervous fluid to the points to be set in activity, that it also creates a flow thither of the fluids of the body, and especially nutrient ones; that, finally, it then places in activity the organs already existing, or makes efforts for the formation of those which would not have existed there, and which a continual want would therefore render necessary—I should have had doubts as to the reality of the law which I have just indicated.
“But, although it may be very difficult to verify this law by observation, I have no doubt as to the grounds on which I base it, the necessity of its existence being involved in that of the third law, which is now well established.
“I conceive, for example, that a gasteropod mollusc, which, as it crawls along, finds the need of feeling the bodies in front of it, makes efforts to touch those bodies with some of the foremost parts of its head, and sends to these every time supplies of nervous fluids, as well as other fluids—I conceive, I say, that it must result from this reiterated afflux towards the points in question that the nerves which abut at these points will, by slow degrees, be extended. Now, as in the same circumstances other fluids of the animal flow also to the same places, and especially nourishing fluids, it must follow that two or more tentacles will appear and develop insensibly under those circumstances on the points referred to.
“This is doubtless what has happened to all the races of Gasteropods, whose wants have compelled them to adopt the habit of feeling bodies with some part of their head.
“But if there occur, among the Gasteropods, any races which, by the circumstances which concern their mode of existence or life, do not experience such wants, then their head remains without tentacles; it has even no projection, no traces of tentacles, and this is what has happened in the case of Bullæa, Bulla, and Chiton.”
In the Supplément à la Distribution générale des Animaux (Introduction, p. 342), concerning the real order of origin of the invertebrate classes, Lamarck proposes a new genealogical tree. He states that the order of the animal series “is far from simple, that it is branching, and seems even to be composed of several distinct series;” though farther on (p. 456) he adds:
“Je regarde l’ordre de la production des animaux comme formé de deux séries distinctes.
“Ainsi, je soumets à la méditation des zoologistes l’ordre présumé de la formation des animaux, tel que l’exprime le tableau suivant:”
In the matter of the origin of instinct, as in evolution in general, Lamarck appears to have laid the foundation on which Darwin’s views, though he throws aside Lamarck’s factors, must rest. The “inherited habit” theory is thus stated by Lamarck.