No. 607] 



RATS AND EVOLUTION 



389 



responsible for the unrestricted feeling of personal lib- 

 erty which systematists undoubtedly have about the way 

 in which they divide a number of dried animals into 

 species. It is for this reason that it here becomes neces- 

 sary first of all to give our definition of the term 

 * ' species. ' ' 



For numerous systematists, a "species" is the descrip- 

 tion of a skin and a skull deposited in a museum— the type- 

 specimen — and to this species belong all the animals which 

 have just such a skin and skull. Some few botanists are 

 just now trying to reserve the term for a group of animals 

 or plants which have the same genotype, the same set of 

 inherited factors of development. As long as we concern 

 ourselves with autogamous plants, such a definition might 

 pass, we might, at least hypothetically, divide a popula- 

 tion of such plants into a number of species and a few 

 hybrid individuals. 



It is very obvious that this definition of "species" falls 

 short, as soon as we concern ourselves with animals, or 

 with allogamous plants. In such groups, according to 

 this definition, there would be no species. Even the geno- 

 typically purest group of animals would in every instance 

 still be composed of two species, the males and the 

 females, for we now know that the sex difference is caused 

 by a difference in genotype.^ Therefore, such a definition 

 of the term although very concise and very short, is prac- 

 tically untenable. 



When we say: Species are those groups of individuals, 

 which have a common genotype, and which are pure for 

 that genotype, we can most certainly concede to Lotsy that 

 species are not variable,- but if wc do so, we limit the use 

 of the old word " species " to those oToups of plants which 

 really are pure and therefore invariable, so tliat they can 

 not be changed by selection, natural or ai t i licial. 



If we solemnly state that do.us liave slioi t twisted tails. 



