In most cases the female lives longer than the male, and this needs no special explanation; but the converse relation is conceivable, when, for instance, the females are much rarer than the males, and the latter lose much time in seeking them. The above-mentioned case of _Aglia tau_ probably belongs to this category. We cannot always decide conclusively whether the life of one sex has been lengthened or that of the other shortened; both these changes must have taken place in different cases. There is no doubt that a lengthening of life in the female has arisen in the bees and ants, for both sexes of the saw-flies, which are believed to be the ancestors of bees, only live for a few weeks. But among the _Strepsiptera_ the shorter life of the male must have been secondarily acquired, since we only rarely meet with such an extreme case in insects. Note 7. Bees. It has not been experimentally determined whether the workers, which are usually killed after some months, would live as long as the queen, if they were artificially protected from danger in the hive; but I think that this is probable, because it is the case among ants, and because the peculiarity of longevity must be latent in the egg. As is well known, the egg which gives rise to the queen is identical with that which produces a worker, and differences in the nutrition alone decide whether a queen or a worker shall be formed. It is therefore probable that the duration of life in queen and worker is potentially the same. Note 8. Death of the Cells in higher Organisms. The opinion has been often expressed that the inevitable appearance of normal ‘death’ is dependent on the wearing out of the tissues in consequence of their functional activity. Bertin says, referring to animal life[25]:—‘L’observation des faits y attache l’idée d’une terminaison fatale, bien que la raison ne découvre nullement les motifs de cette nécessité. Chez les êtres qui font partie du règne animal l’exercise même de la rénovation moléculaire finit par user le principe qui l’entretient sans doute parceque le travail d’échange ne s’accomplissant pas avec une perfection mathématique, il s’établit dans la figure, comme dans la substance de l’être vivant, une déviation insensible, et que l’accumulation des écarts finit par amener un type chimique ou morphologique incompatible avec la persistance de ce travail.’