196 INBREEDING AND 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 physiological 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- 

 zygous characters possible. We have examiner) many 

 populations of this character in the genus Nicotiana and 

 have been astounded at the extremely narrow variability 



