660 RODENTS AND RABBITS xxv. 3-4 



all parts of the world, including in relatively recent times South 

 America and Australasia, reaching the latter before man. They are 

 the only mammals indigenous to New Zealand. The family includes 

 an enormous variety of mice, rats, dormice, field mice, hamsters and 

 so on, and also some special forms such as the jerboa- rat (Notomys) 

 of Australia, which has paralleled the true jerboas in its jumping habits. 

 The rats and mice may be considered as parasites of man and are still 

 changing their distribution. Rattus has been widely spread in recent 

 centuries. The black rat (R. rattus) prefers warmer and drier condi- 

 tions than the brown rat (R. norvegicus) but the two often compete. 



The voles (Cricetidae) are a related family, including partly aquatic 

 as well as terrestrial forms. The field vole (Microtus agrestis) is apt to 

 increase greatly in numbers, producing notoriously destructive plagues. 

 Lemmus, the lemming (Fig. 430), is a mouse-like form living on grasses 

 and roots on the Norwegian mountains. At irregular intervals of 3-5 

 years the population grows greatly by increase in the numbers in the 

 litters and in the number of litters in the season. The population 

 becomes too great for the area to support and large numbers emigrate 

 to the lowlands and die of starvation or from predators. Others reach- 

 ing the sea-shore swim out and are drowned. 



The histricoid rodents are also a large group, with the infra-orbital 

 canal enlarged for the medial part of the masseter, but with the lateral 

 part attached to the zygoma. They include the porcupines (Hystrix) 

 of Africa and Asia (Fig. 431) and the somewhat different porcupines 

 of North America, but all the rest of the group occurs in South 

 America, an area invaded by hardly any other rodents. Fossil histrico- 

 morphs are found in South America from the Oligocene. The 

 Hystricidae all have long spines, used for attack by a rapid backward 

 movement as well as for defence. The cavies and capybaras (Fig. 432) 

 are closely related South American forms, the latter being a large semi- 

 aquatic animal with a greatly enlarged and folded last upper molar. 

 The agoutis (Fig. 434) are also rather large, for rodents, and are 

 terrestrial, often burrowing forms. The vizcachas, Lagostomus (Fig. 

 433), also burrow underground, often making large colonies. 



Possibly related to the hystricomorphs are still more modified digging 

 animals, the Bathyergidae or African mole-rats, which have lost most 

 of the hair and developed long claws on the forelimbs. 



4. Order Lagomorpha 



The rabbits and hares are nowadays considered to be a very isolated 

 offshoot of the early eutherian stock, whose similarities to the Rodentia 



