i<oQ .LAWS OF MULTIPLICATION. 



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 is eaten by a Water-fowl, the reproductive system 

 of the transferred Bothriocephalus 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 in- 

 testine 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 unfavour- 

 able 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, that 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 J^oung shoots, they sit and suck — appropriating the 

 nitrogenous elements of the sap and ejecting its saccharine 

 matter as "honey dew." Along with a sluggishness 

 strongly contrasted with the activity of their allies — along 

 with a very low rate of consumption and a correlative degra- 

 dation of structure ; we have here a retrogression to asexual 

 genesis, and a greatly-increased rate of multiplication. 



The recently-discovered instance of internal metagenesis 

 in the maggots of certain Flies has a like meaning. In- 

 credible as it at first seemed to naturalists, it is now proved that 

 the Cecydomia-\&YY?L develops in its interior a brood of larvae 



