598 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



ferentials to any great extent, as is indicated by the exchange of tissues 

 between these races. But it may be held that certain gradations exist in the 

 relations between the organismal differentials and these organ mutations, 

 which are superimposed upon numerous genetic differences which already 

 exist between different individuals, families and strains. However, in addition 

 the possibility would have to be considered that changes in organismal differ- 

 entials depend on specific mutations, which do not affect one single organ or 

 tissue but certain characteristics common to all organs and tissues. 



The analysis of organismal differentials from the viewpoint of evolution 

 is bound up with the analysis of the genetic differences between races 

 (strains), subspecies, species and genera, and of the mode of origin of these 

 genetic differences and of speciation. As a rule, multiple genetic and chromo- 

 somal differences distinguish races, subspecies, species and genera. These 

 differences are caused by an accumulation of mutations, which consist either 

 in changes taking place in chromosomes or in genes. They occur in a popu- 

 lation which is spread out over a certain geographic area. Subsequent proc- 

 esses of selection, which vary in character in different environments, seem 

 to lead to the formation of geographic races and species and may explain the 

 adaptation which exists between these groups and the environment in which 

 they live; at least quite commonly certain environmental characteristics are 

 associated with certain structural and functional characteristics of the or- 

 ganisms inhabiting certain areas (F. B. Sumner), and these associations 

 between environment and constitution of organisms seem to develop independ- 

 ently in various places and in different races or species belonging to the same 

 wider unit. Such differences between races, species and genera are greatly 

 aided by lack of interbreeding between adjoining populations and these ob- 

 stacles to the interbreeding may be produced by a variety of factors. In case 

 they are due to structural and functional differences in sex organs, such dif- 

 ferences are of no greater importance in the distinction between species than 

 differences in other organs and organ systems ; but the consequences of dif- 

 ferences in sex organs leading to sterility between adjoining populations are 

 much more important as far as speciation is concerned. There remains still 

 the problem as to whether the adaptations noted between environmental con- 

 ditions and structural and functional peculiarities of organisms are caused by 

 random mutations followed by selective processes, or whether, in unknown 

 ways, certain ecologic conditions exert a certain influence on the character of 

 mutations which take place in these environments. 



While the number and nature of structural differences, and in particular, 

 also the interferences with interbreeding between races (strains), species and 

 genera may be taken as indicators of the degree of difference between the 

 organismal differentials of these groups, the real relationship between these 

 organismal differentials can be determined only by the direct tests for or- 

 ganismal differentials and the structure of the latter is the real criterion of 

 the nearness or distance in relationship between individuals and species. As to 

 the character and number of genes which differentiate various species and 

 whole groups of species, we may refer to the investigations of Landsteiner and 



