l62 FOOT-NOTES TO EVOLUTION. 



that not in all forms is this power of the reproduction 

 of the whole organism so sharply limited to the germ 

 cells alone. The familiar propagation of plants by cut- 

 tings, the regeneration of complete animals from small 

 portions of their somatic tissues in many lower forms, 

 and numerous other considerations such as these, show 

 clearly that the difference between the powers of somatic 

 and germinal cells is but one of degree; that while in 

 higher organisms the two seem sharply defined from 

 each other, a series of lower forms may be taken which 

 will show the intermediate steps in this gradual speciali- 

 zation of function. 



In the unicellular organisms we have most interest- 

 ing examples of the fundamental facts of reproduction, 



and through an examination of these 



Reproduction in „ • • • U4. • ^ i-u 



^ ^ we may gam an insight into the more 



Protozoa. ,. j r , 



complicated processes of the Metazoa. 



Each of these lowest forms consists of a single cell in 

 which are carried out in a generalized way the complex 

 physiological functions which, in many celled animals, 

 are divided up among cell groups. In reproduction the 

 animal simply divides into two, the division of the 

 nucleus preceding that of the cytoplasm, and the 

 method is usually a more or less modified karyokinetic 

 one. This mode of multiplication continues in most 

 forms for a certain number of generations, and then the 

 necessity for conjugation — i.e., a temporary or perma- 

 nent fusion with another individual — sets in. If this 



conjugation be prevented, the animal 

 Conjugation. , - , 



' soon shows increasing signs of degen- 



eration which result in death. This " senescence " of 

 the powers of growth and multiplication can only be 

 checked by the admixture of new nuclear substances 

 from an entirely different individual by conjugation. 

 In its simplest terms this process is found in Chilodon, 



