198 SPECIES AND VARIETIES. 



isolation, such individuals do not found strains of their own 

 type, they constitute a variety. Some groups of individuals are 

 so constituted and situated that they constitute one "Paa- 

 rungs-genossenschaft", one association within which all mat- 

 ings are possible. Such groups have a certain variability, a 

 certain number of genes, for which not all the individuals are 

 homozygous. We have called this variability the Total poten- 

 tial variability, and we have given the following definition of 

 what constitutes a species. 



A species is a group of organisms which is so situated and 

 so constituted, that it tends automatically to reduce its total 

 potential variability and which for this reason tends to become 

 pure for one specific type. 



We call a variety those individuals together which differ in 

 some marked way from the common type, when there is 

 nothing in these qualities or in the circumstances, which iso- 

 lates these individuals from crossing freely with the typical 

 ones. 



It will be seen from these definitions, that apparent trivial 

 differences in circumstance may decide whether one or two 

 aberrant individuals will be specifically distinct or will be only 

 a new variety. 



In habitually self-fertilized organisms the course of evolution 

 is fundamentally different from that in allogamous organisms. 

 In autogamous plants there are no varieties. 



In allogamous plants and animals the rule, that species differ 

 in groups of characters and varieties in single ones is of com- 

 mon application. New species can only arise through some 

 sort of isolation, of a group with a potential variability dis- 

 tinct from that of the type. In such a group the total potential 

 variability gradually diminishes, and it becomes relatively pure 

 for its own type, chance only deciding in how many points it 

 will differ from the one or several species from which it origin- 

 ated. Varieties on the other hand are produced by chance 

 combinations of gametes both lacking a certain gene, or supple- 

 menting each others genotype and so giving a new character. 



