378 cook 



stances of Hieracium upon which Nageli based his theory of 

 evolution in a definite direction. The persistence by partheno- 

 genesis of the individual differences of transplanted specimens 

 was accepted as proving that variation held to definite directions. 



Likewise De Vries has made use of linic autogamous species 

 of Draba to illustrate his conception of elementary species. 

 The uniformity and stability of the line-bred plants has been 

 taken to represent the normal condition of species, and the in- 

 ference has been made that the species recognized in nature by 

 taxonomists are generally composed of similar independent 

 units, the effect of the method of propagation, to resolve the spe- 

 cies into separate lines of descent, being left out of consideration. 



Clonic Species. — Species consisting of separate lines of de- 

 scent continued by vegetative propagation alone. 



Clones, like lines, are propagated from single individuals, 

 but by vegetative processes only, so that variation is almost 

 completely avoided. Nevertheless, even vegetatively propa- 

 gated plants are not completely uniform. Clonic groups of the 

 same origin often show fine gradations of diversity, and occa- 

 sional mutative variations are known. 



Clones do not exist, of course, among the higher animals, but 

 they areexceedingly numerous among plants. Several domes- 

 ticated species now exist, as far as known, only in this form. 

 The horse-raddish, sweet-potato, banana, arracacha, yautia and 

 taro may be mentioned as seedless plants, but large numbers of 

 others are nearly seedless or have varieties which are seedless. 



THEORIES OF EVOLUTION BY RESTRICTED DESCENT. 



It is a noteworthy fact that the earlier theories of evolution, 

 including those of Darwin, Nageli, Gulick and De Vries, have 

 been based upon one or another condition of restricted descent. 

 The kinetic theory is the only suggestion of a method of evolu- 

 tion applicable to conditions of unrestricted descent. The pre- 

 disposition to see in restricted descent ideal conditions of evolu- 

 tion has been strengthened, if it has not been wholly supported, 

 by the fact that it is only in restricted descent that the traditional 

 ideal of heredity can be applied. Only narrow-bred organisms 

 afford even an approximate identity of form and structure. 



