Rejuvenescence can be rendered intelligible in theory by some such metaphor, but considerable difficulties are encountered in the rigid application of the metaphor to the facts of the case. In the first place, how is it possible that the motive force can be exhausted by continual division, while one of its components is being formed afresh in the same body and during the same time? When thoroughly examined the loss of the power of division is seen to follow from the loss of the powers of assimilation, nutrition, and growth. How is it possible that such a power can be weakened and finally entirely lost while one of its components is accumulated? I believe that, instead of accepting such daring assumptions, it is better to be satisfied with the simple conception of living matter possessing as attributes the powers of unlimited assimilation and capacity for reproduction. With such a theory the mere form of reproduction, whether sexual or asexual, will have no influence upon the duration of the capacity: for force and matter undergo simultaneous increase, and are inseparably connected in this as in all other instances. This theory does not, however, exclude the possible occurrence of circumstances under which such an association is no longer necessary. I could only consent to adopt the hypothesis of rejuvenescence, if it were rendered absolutely certain that reproduction by division could never under any circumstances persist indefinitely. But this cannot be proved with any greater certainty than the converse proposition, and hence, as far as direct proof is concerned, the facts are equally uncertain on both sides. The hypothesis of rejuvenescence is, however, opposed by the fact of parthenogenesis; for if fertilization possesses in any way the meaning of rejuvenescence, and depends upon the union of two different forms of force and of matter, which thus produce the power of reproduction, it follows that we cannot understand how it happens that the same power of reproduction may be sometimes produced from one form of matter, alone and unaided. Logically speaking, parthenogenesis should be as impossible as that either nitric acid or glycerine should separately produce the effect of nitro-glycerine. The supposition has indeed been made that in the case of parthenogenesis, one fertilization is sufficient for a whole series of generations, but this supposition is not only incapable of proof, but it is contradicted by the fact that certain eggs which may develope parthenogenetically are also capable of fertilization. If, in this case, the power of reproduction were sufficient for development, how is it that the egg is also capable of fertilization; and if the power were insufficient, how is it that the egg can develope parthenogenetically? And yet one and the same egg (in the bee) can develope into a new individual, with or without fertilization. We cannot escape this dilemma by making the