further assumption, which is also incapable of proof, that a smaller amount of reproductive force is required for the development of a male individual than for the development of a female. It is true that the unfertilized eggs of the bee produce male individuals, while the fertilized ones develope into females, but in certain other species the converse association holds good, while in others, again, fertilization bears no relation to the sex of the offspring. Although the mere fact that parthenogenesis occurs at all is, in my opinion, sufficient to disprove the theory of rejuvenescence, it is well to remember that parthenogenesis is now the only method of reproduction in many species (although we do not know the period of time over which these conditions have extended), and is nevertheless unattended by any perceptible decrease in fertility. From all these considerations we may draw the conclusion that the process of rejuvenescence, as described above, cannot be accepted either as the existing or the original meaning of conjugation, and the question naturally arises as to what other significance this latter process can have possessed at its first beginning. Rolph[187] has expressed the opinion that conjugation is a form of nutrition, so that the two conjugating individuals, as it were, devour each other. Cienkowsky[188] also regards conjugation as merely ‘accelerated’ assimilation. There is, however, not only an essential difference but a direct contrast between the processes of conjugation and nutrition. With regard to Cienkowsky’s view, Hensen[189] has well said that ‘coalescence in itself cannot be an accelerated nutrition, because even if we admit that both individuals are in want of nourishment, it is impossible that the need can be supplied by this process, unless one of them perishes and is really devoured.’ In order that an animal may serve as the food of another, it must perish and must be brought into a fluid form, and finally it must be assimilated. In the case before us, however, two protoplasmic bodies are placed side by side and coalesce, without either of them passing into the liquid state. Two idioplasms unite, together with all the hereditary tendencies contained in them; but although it is certain that nutrition in the proper sense of the word cannot take place, because neither of the animals receives an addition of liquid food by the coalescence, yet the consequence of this process must be in one respect similar to that of nutrition and growth:—the mass of the body and the quantity of the forces contained in it undergo simultaneous increase. It is not inconceivable that effects are by this means rendered possible, which under the peculiar circumstances leading to conjugation, could not have been otherwise produced.