214 THE INDUCTIONS OF BIOLOGY. 



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

 Huxley, pseucl-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, there 

 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 only 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-motha 

 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 larvae. 

 In certain Lepidoptera, however, of the groups Psychidcp and 



