No. 607] 



EATS AND EVOLUTION 



403 



common that it must be an exception when a ship trans- 

 ports a number of rats from one port to the other without 

 changing the type. 



The rat population of a frequented port can not be 

 taken as typical for the country where the port happens 

 to be situated. It is always easy to find new tjipes of rats 

 for museum collections in cities having much shipping. 

 But it goes without saying, that such animals, with aber- 

 rant coat characters, aberrant tail length, aberrant type 

 of skull, perhaps, may not be called species without 

 further ado. It is very possible that at the present mo- 

 ment there exist in Sourabaya twelve still undescribed 

 types of rats, which exist nowhere else on Java. But it is 

 probable that after ten years, thirty-five generations, 

 those types will have all made i)l;K'e for an additional 

 dozen of completely different alx-rrant types. 



Such new types have on Java, which is thickly infested 

 by rats, no chance as house-rats, no chance as field-rats, 

 no chance as tree-rats. A little better is the chance which 

 species have, whose habits of life adapt them to a special 

 environment, where they have little or no competition to 

 fear, or at least only from species which have such a geno- 

 type, that they do not mate with the invading species. 



Mus norvegicus does not mate with animals of the Kat- 

 tus group and therefore this species can, without being 

 annihilated, penetrate into a region which is already 

 settled by Rattus rats. We have tried to product hybrids 

 between Mus rattus and Mus norvegicus. We put eight 

 males and ten females of Mus rattus into a large cage, and 

 when we observed the animals mating, we took out the 

 females, and substituted an equal number of Mus norve- 

 gicus females. The males ke]^t on co])nlatinQ-, but al- 

 though we saw numerous apparently noi-inal inatings 

 taking place, we never got a i)re,uiiai)t female. Tliei-e is, 

 as we could observe, no real aiita,u()ni>Jii between Mus 



