128 GENERAL PRINCIPLES OE ZOOLOGY 



oxygen, hydrogen, nitrogen, and sulphur have been brought together to produce 

 living substance. This process is called sptmtaneans generation. If the carbon, 

 oxygen, nitrogen, etc., which are now combined in a stable manner in organisms 

 were formerly unstable, the conditions for the origin of compounds, through 

 whose wider combination life would be possible, may have been more favorable. 

 Thus the hypothesis of the first origin of life through spontaneous generation is 

 carried to a logical postulate. 



II. GENERATION BY PARENTS, OR TOCOGONY. 



We deal here only with those methods of reproduction which 

 have actually been observed, i.e., generation by parents. These methods 

 fall mainly into two great groups, asexual and sexual generation, monogony 

 and amplngony, to which may be added a third group, a combination of 

 the two. 



a. Asexual Reproduction. Monogony. 



Monogony Defined. The chief characteristic of asexual reproduction 

 is the fact that only a single organism is necessary. But since, in certain 

 modes of sexual reproduction (hermaphroditism, parthenogenesis), 

 this also holds true, further explanation is necessary. Asexual reproduc- 

 tion must be a result of the growth of the organism, which has the peculiar- 

 ity that it is not growth for an existing individual, but leads to the forma- 

 tion of new individuals. It is noteworthy in this connection that many 

 animals can reproduce asexually before they have reached the normal 

 size (budding in embryo and larval polyzoa and tunicates). This growth 

 may be general and result in an equal growth of all parts; or it may be 

 local and consequently lead to the formation of an outgrowth in the 

 region of greatest increase. In the first case division takes place, in the 

 latter budding. 



Division. In the case of division (cf. figs. 120, 123, 150) an animal 

 separates into two or more equivalent parts, so that it is not possible to 

 distinguish the mother and the daughter animal, for the original animal 

 has completely disappeared in the young generation. The division 

 is commonly a transverse one, the plane of division being perpendicular 

 to the long axis of the animal; less common is longitudinal division, 

 rarest is oblique. 



Budding. In budding (fig. 93), the products are unequal. One 

 animal maintains the identity of the mother, while the bud, the out- 

 growth caused by local increase, appears as a new formation, as the 

 daughter individual. Yet the difference between division and budding is 

 bridged by intermediate conditions. 



