"It is easy for any one to see that the habit of exercising an organ in every living being which has not reached the term of diminution of its faculties not only makes this organ more perfect, but even makes it acquire developments and dimensions which insensibly change it, with the result that with time it renders it very different from the same organ considered in another organism which has not, or has but slightly, exercised it. It is also very easy to prove that the constant lack of exercise of an organ gradually reduces it and ends by atrophying it." Then follow the facts regarding the mole, spalax, ant-eater, and the lack of teeth in birds, the origin of shore birds, swimming birds and perching birds, which are stated farther on. "Thus the efforts in any direction, maintained for a long time or made habitually by certain parts of a living body, to satisfy the needs called out (_exigés_) by nature or by circumstances, develop these parts and cause them to acquire dimensions and a form which they never would have obtained if these efforts had not become an habitual action of the animals which have exercised them. Observations made on all the animals known would furnish examples of this. "When the will determines an animal to any kind of action, the organs whose function it is to execute this action are then immediately provoked by the flowing there of subtile fluids, which become the determining cause of movements which perform the action in question. A multitude of observations support this fact, which now no one would doubt. "It results from this that multiplied repetitions of these acts of organization strengthen, extend, develop, and even create the organs which are there needed. It is only necessary to closely observe that which is everywhere happening in this respect to firmly convince ourselves of this cause of developments and organic changes. "However, each change acquired in an organ by habitual use sufficient to have formed (_opéré_) it is preserved by generation, if it is common to the individuals which unite in the reproduction of their kind. Finally, this change propagates itself and is then handed down (_se passe_) to all the individuals which succeed and which are submitted to the same circumstances, without their having been obliged to acquire it by the means which have really created it. "Besides, in the unions between the sexes the intermixtures between individuals which have different qualities or forms are necessarily opposed to the constant propagation of these qualities and forms. We see that which in man, who is exposed to such different circumstances which influence individuals, prevents the qualities of accidental defects which they have happened to acquire from being preserved and propagated by heredity (_génération_).