No. 607] 



RATS AND EVOLUTION 



395 



duced by matings between house-rats and house-rats or 

 tree-rats inter se. If the occasional hybrids grow up, 

 they will either become house-rats or tree-rats, biolog- 

 ically speaking. In the first case they will mate with in- 

 dividuals of the house-rat population, in the other case 

 with tree-rats. A new group, so situated that its potential 

 variability is bound to be reduced to produce a genotj^e 

 of its own or a new species, these, a few hybrid rats will 

 certainly not produce. A single mating of a house-rat 

 female with a tree-rat male may be the cause for a height- 

 ening of the potential variability of the house-rat popula- 

 tion into which the hybrids merge. Eventually this higher 

 potential variability will be reduced again. And re- 

 versely, an occasional mating of tree-rat females with • 

 house-rat males may be the cause for a greater potential 

 variability of the group of tree-rats to which the females 

 belong. 



If it so happens that a few animals colonize out of such 

 a population at the time when the potential variability is 

 still higher than ordinarily, such a colony, which will 

 have a potential variability smaller than that of the mul- 

 titude, will have a chance of having a range of variability 

 differing from that of the multitude. Such a group may 

 become pure in respect to a somewhat longer tail, a some- 

 what darker belly or a somewhat greater size, as com- 

 pared to the population from which it ultimately was de- 



Very good examples of such a process can easily be 

 found by observing the evolution of certain species of 

 dogs or poultry under domestication. For instance, the 

 species Airedale terrier has become variable, and there- 

 fore liable to the influence of selection in different direc- 

 tions, because of the fact that hybrids with Dobermann 

 pincher in Germany, and with the Gordon setter in Eng- 

 land, have been taken up into the species, the stud not 

 having been closed rigorously, such as the Sloughi stud, 

 or the Jersey cattle stud. But it must not be thought 

 that a new, improved species of Airedale terrier has been 



