A. B. Stout 123 



simulates a condition of so-called enforced hybridity or heterozygosity 

 involving selective elimination of the homozygotes. In making such a 

 ratio, however, many grades of self-fertility are classed together. The 

 experimental evidence that such ever-sporting races of inbred lines of 

 descent are really hybrids is often obtained by the very questionable 

 method of crossing with some other race. Continued variability and 

 reversibility of characters in inbred lines are best interpreted as marked 

 deviations in quantitative values or potencies of the ultimate units 

 which Mendelian analysis and description may give. In fact such 

 variations are very generally recognized by students of heredity. 



2. Incompatibilities do not arise in species as a condition induced 

 by self-fertilization and inbreeding. 



The question as to the cause of variability in the compatibilities in 

 hermaphrodites, and of the origin of sexual incompatibilities and their 

 significance in evolution, involves in some measure at least the more 

 immediate question of their relation to inbreeding and cross-breeding, 

 and of the relative fertility of hermaphrodites of self-bred and cross-bred 

 parentage. 



It seems necessary to reiterate that Darwin was consistent in his 

 repeated interpretations that what he called self- sterility (the type due 

 to physiological incompatibility) is an incidental and sporadic condition 

 arising from the influence of environment on the constitution of the sex 

 elements. He specifically rejected (1876, p. 345) the view that such a 

 condition arises through physiological results of inbreeding, or that it 

 involves a fundamental necessity for cross-fertilization. He did not 

 consider that it is a condition acquired for the special advantage of 

 preventing self-fertilization. 



Darwin held that the physiological conditions operating in the self- 

 sterile plant involve a lack of differentiation ; the sex organs were 

 considered to be too much alike in constitution. Most writers have 

 sought to explain self-incompatibility on this basis ; either on the basis 

 of similarity of cytoplasmic constitution (Morgan, 1904, 1910), or of 

 hereditary units of germ plasm either of direct influence (Correns, 1912), 

 or of indirect influence (East, 1915), or of hereditary value in trans- 

 mission but cytoplasmic in the immediate relations of fertilization 

 (East and Park, 1918). East and Park have expressed the view that 

 cross-incompatibilities at least are decidedly increased by inbreeding. 



In considering the fertility of any stock one readily recognizes with 

 Darwin (1876, p. 312) that there are involved (1) the production of 

 perfectly formed sex organs and (2) the relative functioning of the organs 



