Factors in Evolution 21 



seed plants, where a single somatic cell can develop 

 into a new plant; but in some liverworts this is 

 possible, and among the algae it is a common phe- 

 nomenon. These asexual reproductive cells in the 

 latter plants are often free swimming " zoospores." 

 This great regenerative power shown by the vege- 

 tative cells of plants is entirely in harmony with 

 the generally lower degree of specialization shown 

 by plants when compared with animals. 



The capacity of reproducing from a single cell, 

 the egg, the whole of an exceedingly complex or- 

 ganism, shown especially in the case of the higher 

 animals, has led to much speculation concerning the 

 actual structures that are the basis of hereditary 

 transmission. The theory of a special " germ- 

 plasm," " biophores," " pangenes," etc., which are 

 the bearers of hereditary characters, has been the 

 subject of many ingenious speculations, none of 

 which, however, is capable of actual demonstration. 

 The phenomena of nuclear division point to the 

 chromosomes as being important agents in heredity, 

 but it is not likely that they are the sole bearers 

 of hereditary characters. 



The Agents in Heredity. The immensely com- 

 plex structure of the protoplast permits of infinite 

 variation in the arrangement of its constituents, and 

 whether or not we assume the presence of special 

 permanent determining structural units such as the 

 " determinants " of Weismann, it is evident that the 

 germ cells of every organism possess their own in- 



