1909] CURRENT LITERATURE 467 
for the phenomenon of repulsion between two dominant factors or units, as for 
example when blueness and the erect standard in peas cannot be combined in the 
same individual, but are found to repel one another. The further work with 
stocks has mainly had reference to the phenomenon of doubling. The doubled 
Stocks are always completely sterile and cannot be used for breeding purposes, 
so that it is not easy to determine the significance of this phenomenon. It is 
found that single stocks are of two kinds, one of which breeds true to the single, 
the other of which always throws a certain proportion of doubles when mated with 
its own kind, and the conclusion is drawn that this second type of single stock 
is perhaps a heterozygote in which the single character is dominant to the double. 
In stocks a difference is also found in the constitution of the pollen grains and ovules 
on the same plant. The presence and absence hypothesis is definitely accepted, 
and Miss DurHam® in another paper appended to the report shows that on 
basis of ‘presence and absence,” the difficulties which Cuénor had found in 
certain mouse crosses completely disappear. 
In a discussion of the ‘‘presence and absence” hypothesis the reviewer? has 
shown that the absence of a character may be dominant over its presence quite 
as well as the reverse. A simple experiment to illustrate this among the other 
Salient features of Mendelian inheritance has also been described.® 
He has also published the results? of his investigation into the elementary 
species and hybrids of Bursa Bursa-pastoris and B. Heegeri, showing that the 
leaf characters of four elementary forms which were found in nature behave in 
the typical Mendelian fashion on crossing with one another. The same relation 
is found to exist between B. Bursa-pastoris and B. Heegeri that exists among the 
several elementary components of the former species, thus giving new proof of the 
derivation of the latter species from the former. However, the Heegeri type of 
capsule which disappears in the first generation of the hybrids reappears in the 
second generation in a ratio of about 22:1. The important feature of these 
results for evolution is the fact that four pure-breeding elementary species of B. 
Heegeri resulted from the cross, when only one had been found before. 
Correns'° has studied variegation and yellow-green foliage in Mirabilis, 
Urtica, and Lunaria, in which he shows that the yellow-green form, which he 
—_—— 
® DurHaAM, FLORENCE H., A preliminary account of the inheritance of coat-color 
in mice. Reports to the Evolution Committee 4:41~63. 1908. 
7 SHULL, G. H., The “ presence and absence”’ hypothesis. Amer. Nat. 43:410-419. 
Tgog 
§—-——_ A simple chemical device to illustrate Mendelian inheritance. Plant 
World 12:145-153. 1909. 
9——_, Bursa Bursa-pastoris and Bursa Heegeri: 
Carnegie Institution of Washington, Publ. 112. pp. 57. igs. 23- pls. 4 
10 CorRENS, C., Vererbungsversuche mit blass (gelb) griinen sii Sel ae 
Sippen bei srabilie Jalapa, Urtica pilulifera, und Lunaria annua. Zeitschr. Abstam. 
Vererb. 1: 291-329. figs. 2. 1909. 
sted = hybrids. 
