ALTERNATION OF GENERATIONS. 



197 



spicuous 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 : — 



Nutrition Reproduction. 



A A 



Anabolism. Katabolism. Female. Male. 



Kic. 76. 



Although it has just been shown that the process of alternation 

 demands a much more thorough analysis and discrimination of the 

 different cases than has hitherto been customary, and this on the 

 physiological as well as merely on the morphological 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 consid- 

 eration of the physiological distinctions between the asexual and 

 sexual generations, shows that the former is the expression of favorable 

 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 propa- 

 gating itself by shoots and runners, and just as an aphis in artificial 

 summer may for years reproduce parthenogenetically, so a hydroid 

 with abundant food and otherwise favorable environment may be 

 retained for a prolonged period vegetative and asexual, while dearth 

 of food and otherwise altered 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 

 favorable to the continuance and preponderance of anabolic processes, 

 and an organism in an environment where katabolism is, at an early 

 stage, likely to gain the ascendant. The former is thus naturally 

 asexual, the latter sexual. A survey, in fact, of the conditions and 

 characteristics of the two sets of forms, inevitably leads us to regard 

 the asexual generation as the expression of predominant anabolism, 



