211 XIIB INDUCTIONS OF BIOLOGY. 



■word ; but rather, as they have since been called by Prof. 

 Huxley, pseud-ova. Yon Siebold and other naturalists, have 

 hence applied the term parthenogenesis to a" narrower class 

 of cases. Perhaps it would be best to distinguish this 

 process, which is intermediate between metagenesis and 

 parthenogenesis, by the term pseudo-parthenogenesis. It 

 is the process familiarly exemplified in the Aphides. 

 Here, from the fertilized eggs laid by perfect females, thero 

 grow up imperfect females, in the pseud-ovaria of which 

 there are developed pseud-ova ; and these, rapidly assuming 

 the organization of other imperfect females, 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. Respecting 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 

 while other branches are still producing onlj^ leaf-bearing 

 axes ; and in the successive generations of Aphides, a parallel 

 truth has been observed. Lastly has to be set down, 



that form 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. In the 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 larvaj. 

 In certain Lcpidoptera, however, of the groups Psychidw. and 



