GENERAL DISCUSSION. 89 



In 1905, he extended somewhat this use of present and absent characters, 

 k (keine) preceding the symbol of a character as a negative. Still he did 

 not pretend to generalize the relation of dominance and recessiveness to 

 be that of presence and absence. In 1903 (p. 146) de Vries stated that in 

 very many cases Mendel's law held when one quality is active and the other 

 latent, and that the active quaUty is dominant. His illustrations show 

 that by activity he meant essentially presence, by latency absence from 

 the visible soma. Bateson's third report (1906) applies presence and absence 

 to several additional cases, and, at the International Genetics Conference 

 of that year. Hurst developed the presence-and-absence hypothesis, favoring 

 the -view that the factor for absence is nothing at all, but finding that certain 

 cases, such as Angora coat, offer a difficulty. At the same meeting I sug- 

 gested that " a variation * * * that is due to abbreviation of the onto- 

 genetic process, which depends on something having dropped out, will be 

 recessive," a progressive variation dominant; and in 1908 I expressed 

 the conclusion that " dominance in heredity appears when a stronger deter- 

 miner meets a weaker determiner in the germ. The extreme case is that in 

 which a strong determiner meets a determiner so weak as to be practically 

 absent, as when a red flower is crossed with white." I suggested that in 

 some cases of recessiveness of an apparent advanced condition, like Angora 

 hair, the dominant factor is an inhibitor. In the last year or two the 

 presence-and-absence theory has gained wide acceptance, but I stiU think 

 the cases where there is dominance of the advanced condition over the less 

 advanced — of the quantitatively well-developed over the quantitatively less 

 well-developed — have not been sufficiently considered. In human hair- 

 color any other hypothesis demands that there are many units in the higher 

 grades of pigmentation and fewer in the lower grades and that the presence 

 of the surplus factor in any other higher grade dominates over its absence 

 in the next lower grade; but there is no evidence in human hair-color of 

 distinct, discontinuous units in the common yellow-brown series. And, 

 in ontogeny, the different grades of color form a continuous series whose 

 development proceeds throughout early life and may even be stimulated 

 to an advanced stage of darkening by disease. The cessation of color develop- 

 ment may take place at any point, and this seems incompatible with the 

 theory of unit-characters for the different grades of human hair-color. In 

 the present paper, on the other hand, the characters dealt with are mostly 

 unit-characters and their quantitative variations mostly heterozygotic. 

 Even the case of the Silkie boot (table 31, C) referred to in an earfier paper * 

 as illustrating recessiveness of the less advanced condition proves, on further 

 analysis, to be a case of heterozygotism. It seems highly probable that the 

 future will show that many more advanced or progressive conditions are 

 really due to one or more unit-characters not present in the less advanced 

 condition. In that case it will appear that there is perfect accord in the two 



* Davenport, 1908, page 60. 



