THEIE VAKIOUS KINDS. 



37 



several representatives being distributed among the in- 

 sectivores, rodents, and egg-laying groups, and the 

 majority pertaining to the two former of these. In fact, 

 in this respect an exact parallelism may be drawn between 

 the mole-like and the spiiny mammals, each assemblage 

 having several representatives among the insectivores and 

 rodents, while the former has a solitary marsupial type, 

 and the latter two members among the egg-laying 

 mammals. Some of these spiny mammals, such as the 

 true porcupines and the echidnas, are burrowing creatures, 



Fig. 15. — Tlie Common Porcupine.* 



and thus have a double means of defence against their 

 enemies ; others, however, lite the hedgehog, rely on their 

 power of rolling themselves up into a ball, and thus pre- 

 senting a chevaux-de-frise on all sides. Some again, like 

 the tree-porcupines, are more or less completely arboreal 

 in their habits ; and the whole of them, like the mole-lite 

 mammals, indicate how urgent has been the need for the 

 lowly-organized rodents, insectivores, and egg-laying 



* We are indebted to Messrs. F. Warne and Co. for the loan of 

 tlie three figures illustrating this chapter. 



