AGAMOGENESIS. 31 



common not only among plants, but among animals of con- 

 siderable complexity. 



The multiplication of flowering plants by bulbs, that of 

 annelids by fission, and that of polyps by budding, are well- 

 known examples of these modes of reproduction. In all 

 these cases, the bud or the segment consists of a multitude 

 of more or less metamorphosed cells. But, in other in- 

 stances, a single cell detached from a mass of such undiffer- 

 entiated cells contained in the parental organism is the foun- 

 dation of the new organism, and it is hard to say whether such 

 a detached cell may be more fitly called a bud or a segment 

 — whether the process is more akin to fission or to gemma- 

 tion. 



In all these cases the development of the new being from 

 the detached germ takes place without the influence of other 

 living matter. Common as the process is in plants and in 

 the lower animals, it becomes rare among the higher animals. 

 In these, the reproduction of the whole organism from a part, 

 in the w 7 ay indicated above, ceases. At most we find that 

 the cells at the end of an amputated portion of the organism 

 are capable of reproducing the lost part ; in the very highest 

 animals, even this power vanishes in the adult ; and, in most 

 parts of the body, though the undifferentiated cells are 

 capable of multiplication, their progeny grow, not into w r hole 

 organisms like that of which they form a part, but into ele- 

 ments of the tissues. 



Throughout almost the whole series of living beings, how- 

 ever, we find concurrently with the process of a gaino genesis, 

 or asexual generation, another method of generation, in which 

 the development of the germ into an organism resembling 

 the parent depends on an influence exerted by living matter 

 different from the germ. This is gamogenesis or sexual gen- 

 eration. Looking at the facts broadly, and without reference 

 to many exceptions in detail, it may be said that there is an 

 inverse relation between agamogenetic and gamogenetic re- 

 production. In the lowest organisms gamogenesis has not 

 yet been observed, while in the highest agamogenesis is ab- 

 sent. In many of the lower forms of life agamogenesis is the 

 common and predominant mode of reproduction, while gamo- 

 genesis is exceptional ; on the contrary, in many of the high- 

 er, w^hile gamogenesis is the rule, agamogenesis takes place 

 exceptionally. 



In its simplest condition, which is termed u conjugation ," 



sexual generation consists in the coalescence of two similar 

 3 



