ASPECTS OK KINETIC EVOLUTION 385 



parental standards, or.at least diverge from them so seriously as 

 to injure the commercial value of the crop, as strikingly shown 

 in the tobacco varieties studied by Mr. A. D. Shamel. 1 



Seed produced by autogamous fertilization yields plants of 

 very much greater uniformity, and it is in this fact that their 

 superiority lies. The plants were not better, as individuals, than 

 some of those produced by the more miscellaneous breeding, but 

 the tendency to degenerate variation had been avoided, or at 

 least postponed. 



Such facts do not appear to warrant any general contrast 

 between cross-fertilization and self-fertilization, but only between 

 narrow breeding and line breeding, and of these the line breed- 

 ing appears to be superior because it constitutes an approxima- 

 tion to vegetative propagation and avoids the need of nuclear 

 readjustments with inadequate diversity of descent. The union 

 of two nuclei which are the autogamous progeny of the same 

 individual organism, can hardly require any new adjustments 

 to be made. The formalities of sexual reproduction are ob- 

 served, but diversity of descent, which gives physiological value 

 and evolutionary significance to the process, has been eliminated. 

 Self-fertility and parthenogenesis, like vegetative propagation, 

 have value only as means of avoiding, for a time, the normal 

 results of restriction of descent, not because they represent 

 normal evolutionary methods of organic succession. 



DIVERSITY REACTIONS IN RESTRICTED DESCENT. 



Efforts toward the selective improvement of domesticated 

 plants and animals have been accompanied everywhere by the 

 narrowing of the lines of descent, and often by close inbreed- 

 ing. How far this abnormal condition is responsible for the 

 results of experiments with domesticated species, and how far 

 these results are of general evolutionary significance, remains 

 to be considered. Most of our important food-plants were 

 domesticated long before the period covered by human history 

 or tradition, so that the general claim of selective improve- 

 ment through thousands of years could not be denied, and has 



'Shamel, A. D., 1906. The Effect of Inbreeding in Plants. Yearbook of U. 

 S. Department Agriculture for 1905, p. 3S6. 



