As has just been said, Lamarck held that the inheritance of acquired characters was only one of the ways in which animals have become changed, and he clearly stated that in the case of all plants and of some of the lower animals the change (evolution) which he supposed them to undergo was due to the general influence of the environment. Since plants and the lower animals (as he supposed) have no central nervous system, or at least no such well-defined nervous system as have the higher animals, Lamarck thought that they could not have evolved in the same way as have the higher animals. We now know that, so far as the lower animals, at least, are concerned, there was no need for such a distinction, since many of their responses are like those of the higher animals. This distinction that Lamarck made is responsible, no doubt, for a misconception that was long held in regard to a part of his views. It is often stated that he supposed the desire of the animal for a particular part has led to the development of that part; while in reality he only maintained the desire to use a particular organ to fulfil some want led to its better development through exercise, and the result was inherited. Lamarck also supposed that the _decrease_ in use of a part which leads to its decrease in size accounts for the degeneration of organs.
Lamarck first advanced his theory in 1801, when he cited the following examples in its favor. A bird, driven through want to the water to find its food, will separate its toes when they strike the water. The skin uniting the bases of the toes will be stretched in consequence, and in this way the broad membrane between the toes of ducks and geese has been acquired. The toes of a bird that is in the habit of perching on a tree become elongated in consequence of becoming stretched, hence has arisen the foot with the long toes characteristic of arboreal birds.
Shore-birds, “which do not care to swim,” but must approach the water in order to obtain food, will be in danger of sinking into the mud, “but, wishing to act so that their body shall not fall into the liquid, they will contract the habit of extending and lengthening their legs.” Hence have arisen the stiltlike legs of shore-birds.