196 INBEEEDING ^ND OUTBREEDING 



tion does not differ from asexual reproduction in 

 what may be called the heredity coefficient. It holds 

 out no advantage as an actual means for the transmis- 

 sion of characters. 



The majority of zoological data on this subject has 

 little value on account of the experimental difficulties in- 

 herent in the material, although zoologists have published 

 more on the matter than the botanists. Plants furnish the 

 best material because of the ease in handling large num- 

 bers of both cuttings and seedlings side by side, and be- 

 cause of the opportunity to utilize hermaphroditic species. 

 Even with the best plant material several undesired vari- 

 ables are present, and experiments with them, therefore, 

 are not without their disappointments ; but no one who 

 has had a long and intimate experience in handling pedi- 

 gree cultures of plants can have any doubts concerning 

 the correctness of our conclusion. Practically the inquiry 

 must take the form of a comparison between the varia- 

 bility of a homozygous race when propagated by seeds 

 and when propagated by some asexual method. The first 

 difficulty is that of obtaining a homozygous race and thus 

 eliminating Mendelian recombination. The traditionally 

 greater variability of seed-propagated strains is due 

 wholly to this difficulty, we believe. It may be impossible 

 to obtain a race homozygous in all factors. There may be 

 a physiolo.2:ical limit to homozygosis even in hermaphro- 

 ditic plants. The best one can do is to use a species 

 which is naturally self-fertilized, relying on continued 

 self-fertilization for the elimination of all the hetero- 

 zyarous characters possible. We have examined many 

 populations of this character in the genus Nicotiana and 

 have been astounded at the extremely narrow variability 



