Thus, while the cells are themselves immortal, the whole organism which they build up is mortal. The complex inter-dependence between the single cells, which, since they have adapted themselves to division of labor, has become necessary, carries with it, from the beginning, the seeds of death. The mutual dependence ceases to work, and the various cells are killed. The death of the individual is a consequence of the defective precision in the working of the division of labor among the cells. This defect, after a longer or shorter time, causes the death of all the cells composing the body. Only those which quit the body retain their power of living. Of all those countless cells which, in the course of a lifetime, are thrown off from the body, only one kind is adapted for existence outside the body, namely, the reproductive cells. Among the lower animals the reproductive cells often leave the body of their parents only after the death of the latter. This is not the case in man. All the cell series which do not take part in the formation of reproductive cells, as well as all the reproductive cells without exception, or with only a few exceptions, die through unfavorable external conditions; just as all, or almost all, of the infusoria which arose from the double cell die before they can conjugate again. At times, however, some of the infusoria persist till the next period of conjugation, and in the same way, from time to time, some of the human reproductive cells succeed in conjugating, and from them a new individual arises. A man is the outgrowth of the double cell produced from the conjugation of two human reproductive cells, and consists of all the cells which arise from this and remain in connection with each other. The human individual originates at the moment of the mingling of the nuclei of the reproductive cells; and the details of this mingling determine his individual peculiarities. The end of man is manifestly to preserve, to nourish, and to protect the series of reproductive cells which are continually developing within him, to select a suitable mate and to care for the children which he produces. His whole structure is acquired by means of selection with this one object in view, the maintenance of the series of reproductive cells.