In Zoologie Philosophique Lamarck argued that form determined function and that the more an organ is used, the larger it grows: “The frequent use of any organ, when confirmed by habit, increases the functions of that organ, leads to its development and endows it with a size and power which it does not possess in animals which exercise it less” (original emphasis; 216). Decreased use has the opposite effect: “The permanent disuse of an organ, arising from a change of habits, causes a gradual shrinkage and ultimately the disappearance and even extinction of that organ” (original emphasis; 210). Zoologie Philosophique provides examples of these laws of evolution and devolution:
In the teeth of the right-whale, which was supposedly completely destitute of teeth, M. Geoffroy has nevertheless discovered teeth in the jaws of the foetus of this animal. The professor has discovered moreover in birds the groove in which the teeth should be placed, although they are no longer to be found there…..The Proteus, an aquatic reptile allied to the salamanders, and living in deep dark caves under the water, has….only vestiges of the organ of sight, vestiges which are covered up and hidden in the same way. 
We find . . . that the bird of the water-side but which does not like swimming and is yet in need of going to the water's edge to secure its prey, is continually liable to sink into the mud. Now this bird tries to act in such a way that its body should not be immersed in the liquid, and thus makes its best efforts to stretch and lengthen its legs. The long established habit acquired by this bird and all its race of continually stretching and lengthening its legs, results in individuals of this race becoming raised as though on stilts, and gradually obtaining long, bare legs, denuded of feathers up to the thighs and often higher still. [216-17]
Lamarck is often incorrectly said to have claiming that the giraffe grew long legs and neck in order to browse the leaves of trees. In reality this was a caricature based on Charles Lyell's comment in volume II of his Principles of Geology. Lamarck actually wrote the following:
It is interesting to observe the result of habit in the particular shape and size of the giraffe (Camelo-pardalis): this animal….is known to live in the interior of Africa where the soil is nearly always arid and barren, so it is obliged to live on the leaves of trees and to make constant efforts to reach them. From this habit long maintained in all its race, it has resulted that the animals fore legs have become longer than its hind legs, and that its neck is lengthened to such a degree that the giraffe, without standing on its hind legs attains a height of 6 meters (nearly 20 feet). [Zoologie Philosophique 219]
He discussed the kangaroo in more detail and noted the ways in which different parts of the body had either grown or reduced in consequence of habitual use or disuse:
This animal, which carries its young in a pouch under the abdomen, has acquired the habit of standing upright, so as to rest only on its hind legs and tail, and of moving only by means of a succession of leaps; during which it maintains its erect attitude in order not to disturb its young. And the following is the result: 1. Its forelegs, which it uses very little and on which it supports itself for only a moment on abandoning its erect attitude, have never acquired a development proportional to that of the other parts, and have remained meagre, very short and with little strength. 2. The hind legs, on the contrary, which are almost continually in action either for supporting the body or for making leaps, have acquired great development and have become very large and strong. 3. Lastly, the tail, which is in this case much used for supporting the animal and carrying out its chief movements, has acquired an extremely remarkable thickness and strength at its base. These well known facts are surely quite sufficient to establish the results of habitual use on an organ or any other part of animals. 
In the same section he described many other examples including the retractable claws of cats, the long necks of geese and some ducks, the positions of eyes on the top of the head in flatfish, the lack of limbs in snakes, the webbed feet of aquatic mammals and birds, and the arms of sloths, many of which were to re-appear in Origin of Species and other volumes by Charles Darwin.
Lamarck was aware that the mummified animals which had been studied in Egypt by Geoffroy St. Hilaire during the Napoleonic expedition showed that little or no change had taken place in these species over two or three thousand years, and that this seemed to contradict his case for change.
"\I do not refuse to believe in the close resemblance of these animals with individual of the same species living to-day…..It would indeed be very odd if it were otherwise…..Indeed there is nothing in the observation now cited which is contradictory to the principles which I have set forth on this subject; or which proves that the animals have existed in nature for all time; it proves only that they inhabited Egypt two or three thousand years ago; and any man who has any habit of reflection and at the same time of observing the monuments of nature's antiquity will easily appreciate the import of a duration of two or three thousand years in comparison with it.
Hence we may be sure that this appearance of the stability of the things in nature will by the vulgar always be taken for reality…….. Everything seems to him to be stable in the planet which he inhabits…. Magnitudes are relative both in space and time: let man take that truth to heart and he will then be more reserved in his judgements on the stability which he attributes to the state of things that he observes in nature. [42-43]
He was also aware of the very long duration of geological time:
Every qualified observer knows that nothing on the surface of the earth remains permanently in the same state. Everything in time undergoes various mutations, more or less rapid depending on the nature of the object and the various conditions…… Naturalists who did not perceive the changes undergone by most animals in course of time tried to connect the facts connected with fossils, as well as the commotions known to have occurred in different parts of the earth's surface, by the supposition of a universal catastrophe which took place on our globe. They imagined that everything had been displaced by it, and that a great number of species then existing had been destroyed.
Unfortunately this facile method of explaining the operations of nature, when we cannot see their causes, has no basis beyond the imagination which created it, and cannot be supported by proof…… in all nature's works nothing is done abruptly, but that she acts everywhere slowly and by successive stages; and on the other hand that the special or local causes of disorders….can account for everything that we observe on the surface of the earth, while still remaining subject to nature's laws and general procedures. [45, 46]
From these principles he concluded that evolution of species had taken place and that the fossil record provided the necessary evidence to prove it.
Lamarck and Man
Others before Lamarck had suggested that mankind and the great apes shared a common ancestor, but he was the first to compare the anatomical features of the skeleton of a chimpanzee with that of a human, and in 1815 concluded that they did share a common ancestry. This shocked many at the time, not least Georges Cuvier and most educated people in France and the rest of Europe, who strongly opposed the idea because it undermined Christian teaching on the uniqueness of mankind among animals. When William Lawrence published very similar views in 1819 in his Lectures on Comparative Anatomy he was forced to withdraw it from sale because of the hostile reaction from those in power who threatened to remove him from his posts as a doctor at Bridewell and Bethlem hospitals. His book, which was published many times in the form of pirated editions, was read by medical students at Edinburgh in the 1830s when Charles Darwin attended lectures there. The controversy concerning the common ancestry of mankind and the great apes continued into the middle decades of the nineteenth century when the leading British anatomist Richard Owen clashed with Thomas Huxley, including at the Oxford meeting of the British Association for the Advancement of Science in 1860. The dispute was not resolved until the middle of the 1860s when Huxley and others were able to finally prove that the anatomy of the great apes and mankind really did point to a common descent from earlier primates.
Lamarck was the first modern scientist to state the theories of the reality and history of evolution and he made an important contribution to the ecological theory on the processes of evolution. It is a common misperception that the theory of evolution is a theory on the process of evolution: it is not because there are many processes which cause and contribute to it. According to S. Lovtrup, an eminent embryologist: "No other person ever made a greater contribution to the study of evolution" than J. B. Lamarck.
Last modified 24 January 2017