290 SHULL GERMINAL ANALYSIS [April 23, 



"insoluble residue." Although Mendelian behavior has proved to 

 be nearly universal in those sexually produced plants and animals 

 which are capable of breeding together normally, there are certain 

 clear limitations to the process of analysis. Several instances are 

 known in which differential characters are not segregated, and no 

 analysis takes place with respect to these characters, even when 

 most of the differential characters of the same plants or animals 

 Mendelize in a perfectly typical manner. The relative frequency 

 of this type of behavior may be greater than is now supposed but 

 so far as clear evidence is available permanently blended inheritance 

 of this type is relatively rare except in species-crosses, and in these 

 latter the data -is usually too scanty for safe generalization. 



Aside from these cases which show a distinctly non-Mendelian 

 mode of inheritance, it must be remembered that Mendelian analysis 

 can be made only in the presence of differential unit-characters 

 possessed by individuals capable of life and of sexual reproduction, 

 and that therefore, there can be no test, except under rare circum- 

 stances, of the Mendelian nature of the more fundamental vital 

 characters. This leaves it an open question whether the whole of 

 the germ-plasm is a complex of such genes as those which give rise 

 to the phenomena of unit-characters, or whether, with its wonderful 

 general powers of assimilation, growth and reproduction, it consists 

 of a great nucleus of which the genes are relatively superficial 

 structural characteristics. 



However, although nothing inconsistent with life and repro- 

 duction are ordinarily amenable to Mendelian analysis, this need not 

 detract from the fundamental importance of unit-characters in the 

 study of heredity and evolution, for the phenomena appearing in 

 these fields are subject to exactly the same limitations. All that we 

 know about heredity and evolution must start with a plasma capable 

 of life and reproduction. 



While thus leaving the absolutely fundamental characteristics of 

 living matter untouched, the Mendelian method and its results have 

 brought into harmonious relations many of the most diverse phe- 

 nomena of phylogenetic differentiation and it is only fair to assume 

 that they hold still greater promise for the future. 



