490 LAWS OF MULTIPLICATION. 



develops reproductive structures, only in some predatory 

 animal to which its first host falls a sacrifice. Occasionally, 

 however, the egg gives origin to the sexual form in the 

 animal that originally swallowed it, but the development 

 remains incomplete there is no sexual genesis, no formation 

 of eggs in the rudimentary segments. That these may 

 become fertile it is needful, as before, for the containing 

 animal to be devoured; so that the imperfect Tape- worm 

 may find its way into the intestine of a higher animal. 

 Thus the Bothriocephalus solidus, found in the abdominal 

 cavity of the Stickleback, is barren while it remains there; 

 but if the Stickleback be eaten by a Water-fowl, the re- 

 productive system of the transferred Bothriocephalus (then 

 known as B. nodosus) becomes developed and active. So, too, 

 a kind of Tape-worm which remains infertile while in the 

 intestine of a Mouse, becomes fertile in the intestine of a Cat 

 that devours the mouse. May we not regard these facts as 

 again showing the dependence of fertility on nutrition? 

 Barrenness here accompanies conditions unfavourable to the 

 absorption of nutriment ; and it gives way to fecundity where 

 nutriment is large in quantity and superior in quality. 



359. Extremely significant are those cases of partial 

 reversion to primitive forms of genesis, which occur under 

 special conditions in some of the higher Annulosa. I refer 

 to the pseudo-parthenogenesis and metagenesis in Insects. 



Under what conditions do the Aphides exhibit this strange 

 deviation from the habits of their order? Why among them 

 should imperfect females produce, agamically, others like 

 themselves, generation after generation, with great rapidity? 

 There is the obvious explanation that they get plenty of 

 easily-assimilated food without exertion. Piercing the tender 

 coats of young shoots, they sit and suck appropriating the 

 nitrogenous elements of the sap and ejecting its saccha- 

 rine matter as " honey dew." Along with a sluggishness 

 strongly contrasted with the activity of most insects along 



