132 Varietäten, Descendenz, Hybriden. 



members of which fail to function because of mutual incompatibility 

 of the elements of the two Systems, 



3. Back Grosses with sylvestris give sylvestris and aberrant forms, 

 and of the two the sylvestris alone are fertile and breed true, On 

 the other hand, back crosses with Tabacu>n produce apparently 

 only Tabacmn forms of which some are completely fertile and con- 

 tinue to produce only Tabacum forms. 



On the theoretical side the foUowing conclusions have been 

 drawn and their application indicated: 



1. As a consequence of modern Mendelian developments, the 

 Mendelian factors may be considered as making up a reaction 

 System the elements of which exhibit more or less specific relations 

 to one another. 



2. Strictly Mendelian results are to be expected only when the 

 contrast is between factor differences within a common Mendelian 

 reaction System as is ordinarily the case in varietal hybrids. 



3. When distinct reaction Systems are involved, as in species 

 crosses, the phenomena must be viewed in the Üght of a contrast 

 between Systems rather than between specific factor differences, and 

 the results obtained will depend upon the degree of mutual 

 compatibility displayed between the specific elements of the two 

 Systems. 



4. Sterility in such cases depends upon non-specific incompati- 

 bility displa5^ed between the elements of the Systems involved, and 

 the degree of this sterility depends upon the degree of such incom- 

 patibility rather than upon a certain number of factors concerned 

 in the expression of such behavior. 



5. The consequences of the application of such a conception to 

 the complex type of behavior in Oenothera are pointed out, and the 

 Suggestion is specifically made that the type of behavior exhibited 

 by Latnarckiana and its segregants in hybridization may be referred 

 to such complex System interactions. M. J. Sirks (Wageningen). 



Harris, J. A., The application ofcorrelationformulaeto 

 the Problem of varietal differences in disease resis- 

 tance: data from the Vermont experiments with po- 

 tatoes. (American Naturalist. LI. p. 238—244. 1917.) 



Having discussed the value of correlation-constants in experi- 

 ments about disease resistance in different varieties or under diffe- 

 rent methods of culture, the author poses also a question of great 

 biological interest, as well as of practical importance: Do varieties 

 differ in their susceptibility to a specific disease only, or do they 

 differ merely in susceptibility to disease in general? 



Altogether he has worked out 23 of these cross correlations — 

 that is correlations between injury to different organs by the same 

 disease, or to the same organ by different diseases, or to different 

 organs by different diseases — . „Only 4 of these, are exceptions to 

 the rule that varieties which show more than the average amount 

 of injury by one disease will, on the whole show more than the 

 average injury by another disease. No one of these exceptional 

 constants can be considered significant with regard to its probable 

 error. Several of the 19 which indicate the rule may be looked 

 upon as individually trustworthy. Thus notwithstanding the large 

 variations in numerical magnitude incident to small series of data 



