392 THE AMERICAN NATURALIST [Vol. LI 



bers of a group have in common, or for which they are 

 not pure (homozygous), and the variability which this 

 impurity makes possible in the descendants. 



At least ideally, we can express the potential variability 

 of a group of individuals in a number. There certainly 

 exist species with a total potential variability of zero; 

 these are, for instance, the pure lines of certain autogam- 

 ous plants, those species for which Lotsy would like to re- 

 serve the term species altogether. 



We will now try, by the aid of this new term, total poten- 

 tial variability, to give such a definition of the word 

 ''species" that it comprises everything which zoologists 

 and botanists, geneticians and systematists, have vaguely 

 meant by it. Our definition is as follows : 



A species is a group of individuals ivhich is so consti- 

 tuted genotypically and which is so situated, that it auto- 

 matically tends to restrict its total potential variability. 



Every group of individuals which is closed to the ad- 

 mixture of individuals from without, such as the de- 

 scendants of an autogamous plant, the dogs or cattle in 

 an exclusive stud, a ''Paarungsgenossenschaft" of ani- 

 mals or plants bound by a peculiar habitat, has the tend- 

 ency to become purer and purer automatically, and to 

 reduce its variability continually. Species originate, 

 given a certain variability of a group of individuals, 

 through all those agencies separately or in combination 

 which bring a group of individuals (not necessarily a 

 small group) into such conditions that the new group has 

 a tendency to become pure for its own genotype. We can 

 not say in general that species are produced by inbreed- 

 ing, or by isolation, or by a change of habitat, or by 

 colonization, or by selection exclusively. An individual 

 or a group must have a certain amount of potential vari- 

 ability to be able to produce a species, different from the 

 one to which it belongs. 



We know now that the genes themselves are invariable. 

 There remain only very few authors who still believe in 

 the variability of the genes. It is therefore necessary to 



