CROSSING. 79 



much smaller than Norvegicus. They have large prominent 

 ears and eyes, a convex nose, harder, sparser, glossier hair, a 

 long thin tail with a soft, rather loose skin. The skin of the tail 

 is liable to break and slip off just as it does in rats of the 

 rattus group. This never happens in norvegicus animals, which' 

 can safely be caught and held by the tail. The ears of the Jap- 

 anese rats are naked. Their eyes are placed at the sides of the 

 head, and the small sunken eyes placed close together on the 

 top of the flat head of Mus norvegicus individuals, is seen in 

 white and spotted rats only if they are descendants of a cross 

 with wild rats. 



To one who is accustomed to handle living rats of many 

 different species, the difference in disposition is striking. The 

 quality of the coat is also very different. Norvegicus has fine, 

 close hair and an abundance of wooly under-fur, which makes 

 the coat water-proof. 



This difference between the soft, furry coat of the Norway 

 rat and the hairier, glossier, harder coat of the hooded rats 

 becomes very striking if the animals are made to swim. The 

 laboratory rats, if of pure origin, become soaking wet and lose 

 their shape if immersed. Norway rats, including albino 

 "sports" and the yellow wild animals of the strain which were 

 caught in London a few years ago, will dive and come up pract- 

 ically dry, only the outside of the coat being wetted. In fact 

 the ordinary laboratory rats look so much like rats of the Mus 

 rattus group, that it is not surprising, that they have long 

 been regarded as a tame variety of Mus rattus. In cranial char- 

 acters however, the laboratory rats are somewhat closer to 

 norvegicus than to rattus, although the difference in skull 

 between them and wild Norway rats is striking. 



The size, fertility, disposition of the domestic cavies differ as 

 much from those of the different wild species as those same 

 characters in the rats. From analogy it would appear probable, 

 that the variability in the cavy which made possible its devel- 

 opment as a domestic animal, resulted from crosses between 

 wild species. 



