ALTERNATION OF GENERATIONS. 



213 



spore-bearing fern-plant and inconspicuous sexual prothallus 

 is again fundamentally parallel. The notation adopted must 

 have already suggested our fundamental diagram, the different 

 forms of which may be separated out or superposed : — 



SUM OF FUNCTIONS. 



Anabolism. Katabolism. Female. Male. 



Although it has just been shown that the process of alter- 

 nation demands a much more thorough analysis and discrim- 

 ination of the different cases than has hitherto been customary, 

 and this on the physiological as well as merely on the morpho- 

 logical side, the general aspect of the process, in which an 

 asexual form alternates with one or more dimorphic sexual 

 generations, makes it evident that we have here to do in two 

 generations with what is often so obvious in one, — the familiar 

 antithesis between nutrition and reproduction. A consideration 

 of the physiological distinctions between the asexual and sexual 

 generations, shows that the former is the expression of favour- 

 able nutritive conditions resulting in vegetative growth, or at 

 most in asexual multiplication, while the latter is conditioned 

 by less propitious circumstances. Just as a well-nourished 

 plant may continue propagating itself by shoots and runners, 

 and just as an aphis in artificial sunmier may for years repro- 

 duce parthenogenetically, so a hydroid with abundant food and 

 otherwise favourable environment may be retained for a pro- 

 longed period vegetative and asexual, while dearth of food and 

 otherwise altereci conditions evoke the appearance of the sexual 

 generation. The contrast between the deeply-rooted well- 

 expanded fern-plant and the weakly-rooted slightly-exposed 

 prothallus, is obviously that between an organism in conditions 

 favourable to the continuance and preponderance of anabolic 



