260 EVOLUTION AND ANIMAL LIFE 



cells, we must bear in mind 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 cuttings, the re- 

 generation 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 T\all show the intermediate steps 

 in this gradual specialization of function. 



"In the unicellular organisms we have most interesting examples 

 of the fundamental facts of reproduction, and through an examina- 

 tion of these we may gain an insight into the more compUcated 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 

 multipHcation continues in most forms for a certain number of genera- 

 tions, and then the necessity for conjugation — i. e., a temporary or 

 permanent fusion with another individual — sets in. If this conjuga- 

 tion be prevented, the animal soon shows increasing signs of de- 

 generation 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 con- 

 jugation. In its simplest terms this process is found in Chilodon, 

 according to Henneguy. Chilodon is a minute fresh- water infusorian, 

 wliich multiplies for a considerable period of time by transverse di\ds- 

 ion. After a time, however, the physiological necessity for conjugation 

 ensues. The animals having placed themselves side by side in pairs 

 and partly fused together, the nucleus of each individual divides 

 into two portions, one of which passes from each infusor into the other 

 to unite with the half remaining stationary. The two then separate, 

 each having received a half of the nucleus of the other. After thus 

 trading experiences, as it might be termed, a period of renewed vigor 

 and activity for each sets in, manifested in rapid growth and multi- 

 plication by division, producing a large number of generations, which 

 continues until weakening 'vital activities indicate the periodically 

 recurring necessity for conjugation. In general, among the Infusoria 

 we find the same process taking place in regular cyclical order, with 



