1 72 GENERAL PRINCIPLES OF ZOOLOGY. 



example all fishes, continue to grow after the beginning 

 of sexual maturity, until they are double or man}- times 

 their size at that time. Sexual reproduction is not even a 

 special form of growth, but a complete renewal of the 

 organism, a rejuvenescence of it. By this is explained 

 the important phenomenon that asexual reproduction is 

 suppressed more and more by sexual reproduction ; the 

 higher the organization of the animal the more the vital 

 energies of its cells must be employed to meet the in- 

 creasing demands upon their functional capacity. 



c. Combined Modes of reproduction. 



Occurrence in the Same Species. Very often there 

 occur in one and the same species of animal two modes of 

 reproduction side by side. Many corals and worms have 

 the power of increasing by division or budding, and also of 

 forming eggs and spermatozoa; again others have no 

 asexual reproduction, but their eggs develop according to 

 circumstances, either parthenogenetically or after fertiliza- 

 tion. The appearance of two kinds of reproduction is very 

 often governed by the fact that individuals with different 

 modes of reproduction alternate in a quite definite rhythm 

 with each another. Such a development is called alterna- 

 tion of generations in the wider sense ; and of this, two 

 special forms are distinguished : metagenesis, or alternation 

 of generations in the narrower sense (progressive alterna- 

 tion of generations), and Jictcrogony (regressive alternation 

 of generations). 



Progressive Alternation of Generations. Meta- 

 genesis. Alternation of generations in the narrower sense, 

 or metagenesis, is the alternation of at least two generations, 

 of which one increases only asexually, by division or bud- 

 ding, the other sexually either exclusively, or at least 

 preponderatingly. The first generation is called the nurse, 

 the second the sexual animal. The reproduction of hydro- 

 mediisce furnishes the best example (Fig. 88). The nurses 

 here are the polyps, which, usually in numbers, united with 



