466 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 liko 

 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 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-\a.YV&. develops in its interior a brood of larvaa 



