"If the duration of human life only extended to the length of a _second_, and if there existed one of our actual clocks mounted and in movement, each individual of our species who should look at the hour-hand of this clock would never see it change its place in the course of his life, although this hand would really not be stationary. The observations of thirty generations would never learn anything very evident as to the displacement of this hand, because its movement, only being that made during half a minute, would be too slight to make an impression; and if observations much more ancient should show that this same hand had really moved, those who should see the statement would not believe it, and would suppose there was some error, each one having always seen the hand on the same point of the dial-plate. "I leave to my readers all the applications to be made regarding this supposition. "_Nature_, that immense totality of different beings and bodies, in every part of which exists an eternal circle of movements and changes regulated by law; totality alone unchangeable, so long as it pleases its SUBLIME AUTHOR to make it exist, should be regarded as a whole constituted by its parts, for a purpose which its Author alone knows, and not exclusively for any one of them. "Each part necessarily is obliged to change, and to cease to be one in order to constitute another, with interests opposed to those of all; and if it has the power of reasoning it finds this whole imperfect. In reality, however, this whole is perfect, and completely fulfils the end for which it was designed." The last work in which Lamarck discussed the theory of descent was in his introduction to the _Animaux sans Vertèbres_. But here the only changes of importance are his four laws, which we translate, and a somewhat different phylogeny of the animal kingdom. The four laws differ from the two given in the _Philosophie zoologique_ in his theory (the second law) accounting for the origin of a new organ, the result of a new need. "_First law_: Life, by its proper forces, continually tends to increase the volume of every body which possesses it, and to increase the size of its parts, up to a limit which it brings about. "_Second law_: The production of a new organ in an animal body results from the supervention of a new want (_besoin_) which continues to make itself felt, and of a new movement which this want gives rise to and maintains. "_Third law_: The development of organs and their power of action are constantly in ratio to the employment of these organs. "_Fourth law_: Everything which has been acquired, impressed upon, or changed in the organization of individuals, during the course of their life is preserved by generation and transmitted to the new individuals which have descended from those which have undergone those changes." In explaining the second law he says: