A. B. Srovur 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 quéstion 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 
