He observed also the effects of use and disuse upon the development of organs: the exercise of an organ leading to its greater development, and the disuse to its degeneration. Numerous illustrations are cited by Lamarck which serve to make his meaning clear. The long legs of wading birds are produced and extended by stretching to keep above the water; the long neck and bill of storks are produced by their habit of life; the long neck of the giraffe is due to reaching for foliage on trees; the web-footed birds, by spreading the toes when they strike the water, have stimulated the development of a membrane between the toes, etc. In the reverse direction, the loss of the power of flight in the "wingless" bird of New Zealand is due to disuse of the wings; while the loss of sight in the mole and in blind cave animals has arisen from lack of use of eyes. The changes produced in animal organization in this way were believed to be continued by direct inheritance and improved in succeeding generations. He believed also in a perfecting principle, tending to improve animals--a sort of conscious endeavor on the part of the animal playing a part in its better development. Finally, he came to believe that the agencies indicated above were the factors of the evolution of life. His Theory of Evolution.--All that Lamarck had written before he changed from botany to zoölogy (1794) indicates his belief in the fixity of species, which was the prevailing notion among naturalists of the period. Then, in 1800, we find him apparently all at once expressing a contrary opinion, and an opinion to which he held unwaveringly to the close of his life. It would be of great interest to determine when Lamarck changed his views, and upon what this radical reversal of opinion was based; but we have no sure record to depend upon. Since his theory is developed chiefly upon considerations of animal life, it is reasonable to assume that his evolutionary ideas took form in his mind after he began the serious study of animals. Doubtless, his mind having been prepared and his insight sharpened by his earlier studies, his observations in a new field supplied the data which led him directly to the conviction that species are unstable. As Packard, one of his recent biographers, points out, the first expression of his new views of which we have any record occurred in the spring of 1800, on the occasion of his opening lecture to his course on the invertebrates. This avowal of belief in the extensive alteration of species was published in 1801 as the preface to his _Système des Animaux sans Vertèbres_. Here also he foreshadowed his theory of evolution, saying that nature, having formed the simplest organisms, "then with the aid of much time and favorable circumstances ... formed all the others." It has been generally believed that Lamarck's first public expression of his views on evolution was published in 1802 in his _Recherches sur l'Organisation des Corps Vivans_, but the