Lamarck: Philosophical ZoologyAdd
 During the last few days of June 1809 the menagerie of the Museum of Natural History received a seal known under the name of sea-calf (Phoca vitulina) which was sent alive from Boulogne; and I had an opportunity of observing the movements and habits of this animal. Thereupon I acquired a still stronger conviction that this amphibian is much more allied to the unguiculate mammals than to the other mammals, notwithstanding the great differences in general shape between it and them.
Its hind legs, although very short like the fore-legs, are quite free and separate from the tail, which is small but quite distinct, and they can move easily in various ways; they can even grasp objects like true hands. 
I noticed that this animal is able to unite its hind feet as we join our hands, and that on then separating the digits between which there are membranes, it forms a fairly large paddle, which it uses for traveling about in the water in the same way as fishes use their tail as a fin
This seal drags itself about on the ground with some speed by means of an undulatory movement of the body, and without any help from its hind legs, which remain inactive and are stretched out. In thus dragging itself about, it derives help from its fore-legs only by supporting itself on the arms up to the wrists, without making any special use of the hands. It seizes its prey either with its hind feet or with its mouth, and although it sometimes uses its hands to rend the prey that it holds in its mouth, these hands appear to be used principally for swimming or locomotion in the water. Finally, as this animal often remains under water for a longish time and even feeds there in comfort, I have noticed that it easily and completely closes its nostrils just as we close our eyes; this is very useful to it when immersed in the liquid that it inhabits.
As this seal is well known, I shall give no description of it. My purpose here is simply to remark that the amphibians have their hind legs set on in the same direction as the axis of their body, for the simple reason that these animals are compelled to use them habitually as a caudal fin by uniting them and by separating the digits so as to form a large paddle. With this artificial fin they are then able to strike the water either to the right or left, and thus move rapidly in various directions.
The two hind legs of seals are so often united and used as a fin that they would not simply have this backward direction in continuance of the body but would be permanently united as in the walruses were it not for the fact that the animals in question also use them very frequently for seizing and carrying off their prey. Now the special movements required by these actions prevents the hind legs of seals from becoming permanently united, and only allow them to be joined together momentarily.
Walruses, on the contrary, which are accustomed to feeding on grass, which they come and browse on the shore, only use their hind legs as a caudal fin; so that in most of them these legs are permanently united with one another and with the tail, and cannot be separated.
We find here a new proof of the power of habit over the form and state of the organs, a proof that I may add to all those already set forth in Chapter VII.
I might add still another very striking proof drawn from mammals. The faculty of flight would seem to be quite foreign to them; yet I can show how nature has gradually produced extensions of the animal's  skin, starting from those animals which can simply make very long jumps and leading up to those which fly perfectly; so that ultimately they possess the same faculty of flight as birds, though without having any affinities with them in their organisation.
Flying squirrels Sciurus volans, aerobates, petaurista, sagitta, volucella) have more recently acquired this habit of extending their wings when leaping, so as to convert their body into a kind of parachute; they can do no more than make a very long jump by throwing themselves to the bottom of a tree, or leaping from one tree on to another at a moderate distance. Now by frequent repetition of such leaps in the individuals of these races, the skin of their flanks is dilated on each side into a loose membrane, which unites the hind-legs to the forelegs and embraces a large volume of air; thus saving them from a sudden fall. These animals still have no membranes between the digits.
The galeopithecus Lemur volans) doubtless acquired this habit earlier than the flying squirrels Pteromis, Geoffr.); the skin of their flanks is still larger and more developed; it unites not only the hindlegs with the fore-legs but also the tail with the hind-legs and the digits with each other. Now these creatures make longer leaps than the preceding, and even perform a sort of flight.
Lastly, the various bats are mammals which probably acquired still earlier than the galeopithecus the habit of extending their limbs and even their digits to embrace a great volume of air, and sustain themselves when they launch forth into the atmosphere.
From these habits, so long acquired and preserved, bats have derived not only lateral membranes but also an extraordinary lengthening of the digits of their four legs (except the thumb) which are united by very large membranes; so that these membranes of the hands, being continuous with those of the flanks and those which unite the tail to the two hind-legs, constitute in these animals great membranous wings with which they fly perfectly as we all know.
Such then is the power of habit: it has a remarkable influence on the shape of the parts and endows animals, which have long contracted certain habits, with faculties not possessed by those which have adopted different habits.
With regard to the amphibians, of which I spoke above, I should like here to communicate to my readers the following reflections that have been raised in me and ever more strongly confirmed by all the subjects I have dealt with in my studies.
I do not doubt that mammals originally came from the water, nor that water is the true cradle of the entire animal kingdom.
We still see, in fact, that the least perfect animals, and they are the  most numerous, live only in the water, as I shall hereafter mention (p. 246); that it is exclusively in water or very moist places that nature achieved and still achieves in favourable conditions those direct or spontaneous generations which bring into existence the most simply organized animalcules, whence all other animals have sprung in turn.
We know that the infusorians, polyps, and radiarians live exclusively in the water; and that some worms even live in it while the rest dwell only in very moist places.
Now the worms appear to form one initial branch of the animal scale, and it is clear that the infusorians form the other branch. We may suppose therefore that such worms as are completely aquatic and do not live in the bodies of other animals, Gordius, for instance, and many others that we are not yet acquainted with, have doubtless become greatly diversified in the water; and that among these aquatic worms, those which afterwards became accustomed to exposure to the air have probably produced the amphibian insects such as gnats, mayflies, etc., etc., while these in turn have given existence to all the insects which live altogether in the air. Several races of these again have changed their habits as a result of their environment and contracted a new habit of living hidden away in solitude: hence the origin of the arachnids, nearly all of which live also in the air.
Finally, those arachnids that frequented water, and gradually became accustomed to live in it until at last they altogether ceased to live in the air, led to the existence of all the crustaceans; this is clearly indicated by the affinities which connect the centipedes with the millipedes, the millipedes with the woodlice, and these again with Asellus, shrimps, etc.
The other aquatic worms, which are never exposed to the air, would have developed in course of time into many different races with a corresponding advance in the complexity of their organization. They would thus have led to the formation of the annelids, cirrhipedes and molluscs, which form together an unbroken portion of the animal scale.
There seems to us to be a great hiatus between the known molluscs and the fishes; yet the molluscs whose origin I have just named have led to the existence of the fishes through the medium of other molluscs that have yet to be discovered, and it is manifest that the fishes again have given rise to the reptiles.
As we continue to examine the probable origin of the various animals, we cannot doubt that the reptiles, by means of two distinct branches, caused by the environment, have given rise, on the one hand, to the formation of birds and, on the other hand, to the amphibian mammals, which have in their turn given rise to all the other mammals. 
After the fishes had led up to the formation of the batrachian reptiles and these to the ophidian reptiles, both of which have only one auricle in their hearts, nature easily succeeded in giving a heart with a double auricle to the other reptiles, which became divided into two separate branches; subsequently she easily achieved the formation of a heart with two ventricles in animals originating from both these branches.
Thus among the reptiles which have a heart with a double auricle, the chelonians appear to have given existence to the birds; for, in addition to their various unmistakable affinities, if I were to place the head of a tortoise on the neck of certain birds, I should find hardly any incongruity in the general appearance of the factitious animals; in the same way the saurians, especially the planicauds, such as crocodiles, seem to have led to the existence of the amphibian mammals.
If the chelonian branch has given rise to the birds, we may suppose that the aquatic palmipeds, and especially the brevipens, such as the penguins and king-penguins, have brought about the formation of the monotremes.
Lastly, if the saurian branch gave rise to the amphibian mammals, it is highly probable that from this branch all the mammals have taken their origin.
I think the belief is justifiable, that the terrestrial mammals originated from those aquatic mammals that we call amphibians. These were divided into three branches by reason of the diversity arising in their habits in the course of time; one of these led to the cetaceans, another to the ungulate mammals, and the third to the various known unguiculate mammals.
Those amphibians indeed which preserved the habit of going on to the beach became divided, owing to their different manner of feeding. Some of them, being accustomed to browsing on grass, as for instance the walruses and manatees, gradually led to the formation of the ungulate mammals such as the pachyderms, ruminants, etc.; the others as, for instance, the seals, having acquired the habit of feeding exclusively on fishes and marine animals, brought about the existence of the unguiculate mammals through the medium of races which as they diversified became altogether terrestrial.
Those aquatic mammals, however, which acquired the habit of never coming out of the water and of only coming to the surface to breathe, probably gave rise to the various cetaceans with which we are acquainted. The cetaceans have been greatly modified in organisation by having dwelt for so long a period exclusively in the sea; hence it is now very difficult to recognize whence they derive their origin.
In consequence of the immense lapse of time during which these animals have lived in the sea without ever using their hind-legs for  grasping objects, these unused legs have entirely disappeared, including their bones, and even the pelvis which served for their support and attachment.
The degeneration in the limbs of cetaceans under the influence of the environment and acquired habit is also seen in their fore-feet, which are entirely invested by skin so as not even to show the digits at the end of them; they thus consist of one fin on each side containing the skeleton of a concealed hand.
Seeing that the cetaceans are mammals, it is assuredly a part of their plan of organisation to have four limbs like all the rest, and consequently a pelvis for the support of their hind-legs. But, here as elsewhere, the loss of these parts is the result of an abortion due to a long disuse of them. When we remember that in seals which still have a pelvis, this pelvis is impoverished, reduced and does not protrude from their haunches, we shall feel that the cause must be the moderate use which these animals make of their hind-legs, and that, if they were to give up using them altogether, the hind-legs and even the pelvis would ultimately disappear.
The arguments which I have just adduced will doubtless seem to be mere guesses, since it is not possible to establish them on direct positive proofs. If we pay attention, however, to the observations set forth in the present work, and if we then closely examine the animals which I have cited and also the effects of their habits and environment, we shall find as a result of this examination that these guesses acquire a high degree of probability.
The table on p. 179 may facilitate the understanding of what I have said. It is there shown that in my opinion the animal scale begins by at least two separate branches, and that as it proceeds it appears to terminate in several twigs in certain places.
This series of animals begins with two branches, where the most imperfect animals are found; the first animals therefore of each of these branches derive existence only through direct or spontaneous generation.
There is one strong reason that prevents us from recognizing the successive changes by which known animals have been diversified and been brought to the condition in which we observe them; it is this, that we can never witness these changes. Since we see only the finished work and never see it in course of execution, we are naturally prone to believe that things have always been as we see them rather than that they have gradually developed.
Throughout the changes which nature is incessantly producing in every part without exception, she still remains always the same in her totality and her laws; such changes as do not need a period much 
Table Showing the Origin of the Various Animals.
 longer than the duration of human life are easily recognized by an observer, but he could not perceive any of those whose occurrence consumes a long period of time.
To explain what I mean let me make the following supposition.
If the duration of human life only extended to one second, and if one of our ordinary clocks were wound up and set going, any individual of our species who looked at the hour hand of this clock would detect in it no movement in the course of his life, although the hand is not really stationary. The observations of thirty generations would furnish no clear evidence of a displacement of the hand, for it would only have moved through the distance traversed in half a minute and this would be too small to be clearly perceived; and if still older observations showed that the hand had really changed its position, those who heard this proposition enunciated would not believe it, but would imagine some mistake, since they had always seen the hand at the same point of the dial.
I leave my readers to apply this analogy to the subject in hand,
Nature--that immense assemblage of various existences and bodies, in all whose parts continually proceeds an eternal cycle of movements and changes controlled by laws--an assemblage that is only immutable so long as it pleases her Sublime Author to continue her existence--should be regarded as a whole made up of parts, with a purpose that is known to its Author alone, but at any rate not for the sole benefit of any single part.
Since each part must necessarily change and cease to exist to make way for the formation of another, each part has an interest which is contrary to that of the whole; and if it reasons, it finds that the whole is badly made. In reality, however, this whole is perfect, and completely fulfills the purpose for which it is destined.
Jean Baptiste LamarckPhilosophical Zoology: An Exposition with Regard to the Natural History of Animals (1809), Part I: Considerations on The Natural History of Animals, Their Characters, Affinities, Organisation, Classification and Species. Translated by Hugh Elliot (1909), scanning and layout by dsb (1996)