GROWTH AND REPRODUCTION. 229 



planarians went on apace when the food supply was copious 

 (anaboHc condition), but if the amount of food was reduced 

 or altogether withdrawn (katabolic condition) the asexual re- 

 production completely ceased. Bergendal reports, that in the 

 transverse division of another planarian worm {Bipalimti)^ the 

 severed links were all sexually immature ; and the results of 

 Rywosch demonstrate the same antithesis between the sexual 

 and the asexual process. 



In the same way, sexual reproduction is contrasted with its 

 degenerate expression in parthenogenesis. The conditions of 

 the latter in aphides and phylloxera are demonstrably anabolic, 

 the normal sexual process recurs with the periodic return of hard 

 times, or in relatively katabolic conditions. In the lower crus- 

 taceans, a similar contrast of conditions has also been observed. 





/ 





Pollen Grain ; a, the two nuclei ; I', the general protoplasm 

 c, the outer wall. — From Carnoy. 



It is again, on the present view, readily intelligible why in 

 the exceptionally favourable anabolic environment of bacteria 

 and many parasitic fungi sexual reproduction should be absent. 

 Marshall Ward has pointed out, that the more intimate the 

 degree of parasitism or saprophytism, the more degenerate the 

 sexual reproduction. The greater the anabolism, in other 

 words, the more growth and the less sexuality. That such 

 comparatively complex organisms can continue their asexual 

 reproduction, dispensing altogether with the acknowledged 

 stimulus of fertilisation, may probably be at least partially 

 explained on the assumption that the abundant waste products 

 of the host act as extrinsic stimuli. 



On this view, moreover, alternation of generations loses 

 much of its uniqueness. The contrast between the vegetative 



