1 68 GENERAL PRINCIPLES OF ZOOLOGY. 



in the increase of individuals and species, where the space 

 is not sufficient for the already more favorably situated 



organisms? 



II. GENERATION BY PARENTS, OR TOCOGONY. 



Two Principal Methods. As mentioned above, we 

 shall 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, mouogony and aiiipJiigoity, 

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

 these two methods of reproduction. 



a. Asexual Reproduction. Monogony. 



Monogony Defined. The chief characteristic of asex- 

 ual reproduction is the fact that for it only a single organ- 

 ism is necessary. But since, in certain modes of sexual 

 reproduction, this same fact also holds true, still further 

 explanation is necessary. Asexual reproduction is pre- 

 pared for by a rapid growth of the organism ; it is, as 

 it has been described, an ovcrgrovvtJi, a grovvtJi beyond tJie 

 individual limits. So soon as an organism grows beyond 

 the definite size suited to it, the material of its body divides 

 into two or more animals. The growth of the organism 

 may be either a general one and lead to a regular increase 

 of the animal in all its parts, or it may be localized and 

 cause a partial increase, and consequently the formation of 

 an outgrowth at the point of greatest increase; in the first 

 case, division takes place, in the latter, budding. 



Division. In the case of division, an animal falls into 

 two or more parts equivalent to one another, 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, in which the plane of division stands per- 

 pendicular to the long axis of the animal ; less common is 

 longitudinal division, rarest is oblique division (the planes 



