Varietäten, Descendenz, Hybriden. 131 



The conclusions to which the author has been driven in this 

 sense, are: 



1. Broad and varied circumstantial evidence indicates unmista- 

 kably that the inheritance of acquired characters has played an 

 extremely important röle in evolution. 



2. Numerous experimental investigations designed to test the 

 possibility of such inheritance directly have either failed utterly or 

 have been open to serious destructive criticism. Direct proof of the 

 inheritance of acquired characters is therefore lacking. 



3. If conclusions 1 and 2 are to be harmonized, either modifications 

 are fully inherited so rarely that proof that they do not belong to 

 the general category of chance changes in Constitution of the germ 

 plasm is impossible, or the imprint of the environment is so weak 

 that extremely long periods of time, perhaps geological epochs, are 

 necessary for its manifestation. 



These conclusions and their connection with the subject at 

 hand, is discussed by the writer. mentioning especially inheritance 

 of mutilations, effects of changed food supply, effects of disease 

 and the work of Cramer and Winkler. 



M. J. Sirks (Wageningen). 



Frost, H. B., The different meanings of the term „fac- 

 tor" as affecting clearness in geneticdiscussion. (Ame- 

 rican Naturalist. LI. p. 244—250. 1917.) 



The writer gives foUowing summary at the end of his paper: 

 The term factor has, in genetic use, two distinct meanings, 

 which are continually interchanged or combined and often confused. 

 It is essential to clearness in genetic discussion that these two mea- 

 nings should be carefully distinguished. These meanings may be 

 indicated by the following formal definitions: 



1. A genetic (Mendelian) factor is a property or characteristics 

 of the germ-plasm, more or less conveniently delimited for the 

 purpose of analysis of segregating heredity. 



2. A genetic (Mendelian) factor or gene, is an actual material 

 Unit of genetic segregation; it is of unknown nature, but probably 

 consists of a genetically indivisible portion of a chromosome (a locus) 

 in a particular State, 



The presence- and absence scheme of factor notation properly 

 employs only the first of these meanings; the Morgan Castle scheme, 

 on the other hand, may use either. M. J. Sirks (Wageningen). 



Goodspeed, T, H. and R. E. Clausen. Mendelian factordif- 

 ferences versus reaction sy stem con trasts in hered ity. 

 (American Naturalist. LI. p. 31-46, 92—101. 1917.) 



Suramarizing briefly the content of their paper, the authors 

 present following facts on the experimental side with reference to a 

 species hybrid: 



1. Nicotiana sylvestris when crossed with various varieties of iV. 

 Tahactirn gives Fi-hybrids which are replicas on a large scale of 

 the particular Tahaciim variety concerned in the cross. 



2. The Fj-hybrids of sylvestris and Tabacum produce a small 

 number of functional ovules which represent the sylvestris and Ta- 

 bacum extremes of a recombination series, the great majority of the 



