212 



THE EVOLUTION OF SEX. 



In the same way, sexual reproduction is contrasted with its degen- 

 erate 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 crustaceans, a similar contrast of 

 conditions has also been observed. 



Fig. 82. — Pollen Grain: a, the two nuclei; l>, the general proto- 

 plasm ; c, the outer wall. — From Carnoy. 



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

 exceptionally favorable 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 acknowl- 

 edged stimulus of fertilization, 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 asexual hydroid 

 or hydra-tuba, and the active sexual medusoid or jellyfish, is very 

 marked. So too, on a higher plane, the vegetative spore-producing 

 fern-plant stands opposed to the less nutritive sexual prothallus. The 

 alternation is but a rhythm of large amplitude between anabolic and 

 katabolic preponderance. 



What is so marked in the alternation is only a specialization of the 

 reproductive or sexual parts of the organism as against the growing or 

 asexual ones, — a specialization which becomes exaggerated into sepa- 

 rate existences, each dominated by its own physiological bias. 



In the fern or flowering plant the vegetative or asexual existence 

 has preponderated, and this is entirely consistent with the character- 



