xxv. 5 FEEDING 663 



is reduced in both groups, but the lagomorphs lack the power of move- 

 ment between the two halves of the lower jaw. 



Taken all together, therefore, the differences are as great as the 

 similarities, even in the gnawing mechanism, which the rabbits share 

 with the other rodents. In the remaining parts there are few points of 

 close similarity, other than those due to the fact that neither set of 

 animals has departed far from the original eutherian condition. More- 

 over, serological studies do not show any signs of closer affinity of the 

 lagomorphs with the rodents than with other mammalian orders. If 

 anything they are more like artiodactyls. However, the lagomorphs 

 share with rodents the habit of passing food twice through the 

 alimentary canal (caecotrophy). Dried faecal pellets are produced only 

 during the day. At night soft pellets covered with mucus are formed 

 in the caecum and are immediately taken from the anus by the lips. 

 They are stored in the stomach and later mixed with further food 

 taken (Fig. 435). The double passage of the food is necessary for 

 the life of mice and guinea pigs as well as rabbits. The animals die 

 in two to three weeks if prevented from reaching the anus. The moist 

 pellets probably contain the metabolites that have been produced by 

 breakdown of cellulose by the bacteria of the caecum, w r hich cannot 

 be absorbed by the organ itself. 



Rabbits and hares have characteristically developed the hind legs 

 for a jumping method of locomotion and there has been a reduction 

 of the tail. The rabbits show a number of specializations for burrow- 

 ing life. They are among the most successful of all mammals, especially 

 in the Holarctic region, but have made relatively little progress in 

 Africa or South America. Their enormous spread in Australia since 

 introduction by man in the eighteenth century shows how accidental 

 limitations of access, and their alteration, affect the distribution of 

 animal life. 



Fossil lagomorphs, quite like modern hares and rabbits, are found 

 back to the Oligocene. Few remains are known from the Eocene, but 

 there is evidence that the type was already distinct in the Palaeocene 

 and has persisted with relatively little change ever since. 



5. Fluctuations in numbers of mammals 



Fluctuation in numbers is characteristic of many rodents and other 

 small mammals. The phenomenon is usually first recorded as a 

 'plague' of the rats, mice, voles, or rabbits, and these may be cases of 

 local and sporadic abundance. Study of some species has shown, how- 

 ever, that there are rather regular cyclical fluctuations in numbers, 



