in a reproductive organ--either an ovarium or the homologue of one--has been called, by Prof. Owen, _parthenogenesis_. It is the process familiarly exemplified in the _Aphides_. Here, from the fertilized eggs laid by perfect females there grow up imperfect females, in the ovaria of which are developed ova that though unfertilized, rapidly assume the organization of other imperfect females, and are born viviparously. From this second generation of imperfect females, there by-and-by arises, in the same manner, a third generation of the same kind; and so on for many generations: the series being thus symbolized by the letters A, B, B, B, B, B, &c., A. Respecting this kind of heterogenesis it should be added that, in animals as in plants, the number of generations of sexless individuals produced before the re-appearance of sexual ones, is indefinite; both in the sense that in the same species it may go on to a greater or less extent according to circumstances, and in the sense that among the generations of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. In trees we see that on some branches flower-bearing axes arise while other branches are still producing only leaf-bearing axes; and in the successive generations of _Aphides_ a parallel fact has been observed. Lastly has to be set down that kind of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. This is called true parthenogenesis--reproduction carried on by virgin mothers which are in all respects like other mothers. Among silk-worm-moths this parthenogenesis is exceptional rather than ordinary. Usually the eggs of these insects are fertilized; but if they are not they are still laid, and some of them produce larvæ. In certain _Lepidoptera_, however, of the groups _Psychidæ_ and _Tineidæ_, parthenogenesis appears to be a normal process--indeed, so far as is known, the only process; for of some species the males have never been found. A general conception of the relations among the different modes of Genesis, thus briefly described, will be best given by the following tabular statement. GENESIS is { Oviparous { or Homogenesis, which is usually Gamogenesis { Ovo-viviparous { or { Viviparous or { Gamogenesis { alternating Heterogenesis, which is { with { Parthenogenesis { Agamogenesis { or { Internal { Metagenesis { or { External