"It is then evident that a great change in circumstances having become constant in a race of animals leads these animals into new habits. "Now, if new circumstances, having become permanent in a race of animals, have given to these animals new _habits_--that is to say, have led them to perform new actions which have become habitual--there will from this result the use of such a part by preference to that of another, and in certain cases the total lack of use of any part which has become useless. "Nothing of all this should be considered as a hypothesis or as a mere peculiar opinion; they are, on the contrary, truths which require, in order to be made evident, only attention to and the observation of facts. "We shall see presently by the citation of known facts which prove it, on one side that the new wants, having rendered such a part necessary, have really by the result of efforts given origin to this part, and that as the result of its sustained use it has gradually strengthened it, developed, and has ended in considerably increasing its size; on the other side we shall see that, in certain cases, the new circumstances and new wants having rendered such a part wholly useless, the total lack of use of this part has led to the result that it has gradually ceased to receive the development which the other parts of the animal obtain; that it gradually becomes emaciated and thin; and that finally, when this lack of use has been total during a long time, the part in question ends in disappearing. All this is a positive fact; I propose to give the most convincing proofs. "In the plants, where there are no movements, and, consequently, no habits properly so called, great changes in circumstances do not bring about less great differences in the development of their parts; so that these differences originate and develop certain of them, while they reduce and cause several others to disappear. But here everything operates by the changes occurring in the nutrition of the plant, in its absorptions and transpirations, in the amount of heat, light, air, and humidity which it habitually receives; finally, in the superiority that certain of the different vital movements may assume over others.