But quite apart from the fact that it is impossible to bring forward any proofs, the theory of rejuvenescence seems to me to be unsatisfactory in other ways. The whole conception of rejuvenescence, although very ingenious, has something uncertain about it, and can hardly be brought into accordance with the usual conception of life as based upon physical and mechanical forces. How can any one imagine that an Infusorian, which by continued division had lost its power of reproduction, could regain this power by forming a new individual, after fusion with another Infusorian, which had similarly become incapable of division? Twice nothing cannot make one. If indeed we could assume that each animal contained half the power necessary for reproduction, then both together would certainly form an efficient whole; but it is hardly possible to apply the term rejuvenescence to a process which is simply an addition, such as would be attained under other circumstances by mere growth; neglecting, for the present, that factor which, in my opinion, is of the utmost importance in conjugation,—the fusion of two hereditary tendencies. If rejuvenescence possesses any significance at all, it must be this,—that by its means a force, which did not previously exist in the conjugating individuals, is called into activity. Such a force would, however, owe its existence to latent energy stored up in each single animal during the period of asexual reproduction, and such latent forces would necessarily be of different natures, and of such a constitution that their union at the moment of conjugation would give rise to the active force of reproduction. The process might perhaps be compared to the flight of two rockets, which by the combustion of some explosive substance (such as nitro-glycerine) stored up within themselves are impelled in such a direction that they would meet at the end of their course, when all the nitro-glycerine had been completely exhausted. The movement would then come to an end, unless the explosive material could have been meanwhile renewed. Now suppose that such a renewal were achieved by the formation of nitric acid in one of the rockets and glycerine in the other, so that when they came into contact nitro-glycerine would be formed afresh equal in quantity and in distribution on both the rockets to that which was originally present. In this way the movement would be renewed again and again with the same velocity, and might continue for ever.