Varietäten, Descendenz, Hybriden. 35 



that hereditary changes consist only in the dropping out of com- 

 plete Units, or that they are bound to occur in large Steps. The 

 „multiple allelomorphs" show that a Single unit factor may exist 

 in a great number of grades; the „multiplying modifying factors" 

 show that a visible character may be modified in the finest 

 gradations by alterations in diverse parts of the germinal material. 

 The objections raised by the mutationists to gradual change 

 through selection are breaking down as a result of the thorough- 

 ness the mutationists own studies. 



The positive contribution of these matters to the selection pro- 

 blem is to enable us to see the important röle played by Mendelism 

 in the effecliveness of selection. Hereditary variations, such as 

 give rise to the multiple allelomorphs and multiple modifying 

 factors, occur in some organisms rather infrequently, as measured 

 by the time scale of human happenings. If there were no inter- 

 change of factors among individuals and Stocks, it would take a 

 long time to obtain in one individual all the six diluters of the 

 eosfn color of the Drosophila eye; one arises in one individual, 

 another in another. But by selective crossbreeding it is possible 

 to bring together into one stock all the modifiers that have been 

 produced in diverse Stocks. Mendelism acts as a tremendous acce- 

 lerator to the effectiveness of selection. 



M. J. Sirks (Wageningen). 



Love, N. A. and A. C. Fräser, The inheritance of the weak 

 awn in certain Avena crosses. (American Naturalist. LI. 

 p. 481—493. 1917.) 



From the data presented in this paper, the following conclu- 

 sions may be drawn as to the inheritance of awns in crosses 

 between the weak-awned (Burt) and the awnless (Sixty-Day) types 

 of oats. The awnless type is almost completely dominant in the 

 first generation, only a few of the plants possessing awns and 

 those in small percentages. 



The second generation gives awnless, partially awned and 

 fully awned plants in a ratio which approximates 1:2:1. The 

 behavior of the fuUy-awned plants show that this type is the pure 

 recessive, for it breeds true in all cases, from the second gene- 

 ration. All of the partially awned Fg plants proved to be hetero- 

 zygous, throwing in the third generation approximately three 

 plants not fully awned to one fully awned plant. 



The awnless plants of the second generation were found to 

 comprise both homozygous plants of the parental type and hetero- 

 zygous intermediates which later broke up in the same manner 

 as the partially awned Fj plants. It might be expected that some 

 of the awnless F2 plants would prove to be heterozygous, since 

 awnless plants are found commonly in the first generation. 



From these results it is apparent. that we cannot correctly 

 speak of the awnless oat as the dominant type, nor should we 

 restrict the use of the term intermediates to those plants which 

 are partially awned. 



It seems very probable that the difference between the weak- 

 awned and the awnless varieties of oats, at least in the varieties 

 studied, may be accounted for by the assumption of a difference 

 in one pair of genetic factors. The data at hand seem to point to 

 the presence of an inhibitor to awning in the awnless variety 



