274 THE INDUCTIONS OF BIOLOGY. 



egg of a Distoma there results a rudely-formed creature known 

 as a sporocyst and from this a redia. Gradually, as this 

 divides and buds, the greater part of the inner substance 

 is transformed into young animals called Cercarice (which 

 are the larvae of Distomata) ; until at length it becomes little 

 more than a living sac full of living offspring. In the Dis- 

 toma jMcifica, the brood of young animals thus arising by 

 internal gemmation are not Cercarice, but are like their 

 parent : themselves becoming the producers of Cercarice, after 

 the same manner, at a subsequent period. So that now the 

 succession of forms is represented by the series A, B, A, B, &c., 

 now by the series A, B, B, A, B, B, &c., and now by A, B, B, 

 C, A. Both cases, however, exemplify internal metagenesis 

 in contrast with the several kinds of external metagenesis 

 described above. That agamogenesis which is carried 



on in a reproductive organ — either an ovarium or the homo- 

 logue 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 de- 

 veloped ova that though unfertilized, rapidly assume the 

 organization of other imperfect females, and are born vivi- 

 parously. From this second generation of imperfect females, 

 there by-and-by arises, in the same manner, a third genera- 

 tion 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. Eespecting this kind of heterogenesis it should be 

 added that, in animals as in plants, the number of genera- 

 tions 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 accord- 

 ing to circumstances, and in the sense that among the genera- 

 tions 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 



