THE SEALS. 291 



insignificance to the organism, were of actual 

 service to their ancestors, just as the deciduous 

 teeth of most of our present mammals are of use for 

 several years. None of these milk-teeth have the 

 prospect of being preserved like the one remaining 

 deciduous tooth of the Marsupials (p. 94) ; accord- 

 ingly the Seals of future periods will undoubtedly 

 not show a trace of milk-teeth. 



The Seals belong to the physically weaker groups 

 of mammals, and it is certainly most remarkable, 

 and as yet not explicable, that the other mammals 

 also, which have already been discussed, and are 

 allied to the Seals as regards the suppression of the 

 change of teeth, belong, on the whole, to the less 

 favoured or less strongly developed orders. For, 

 as we have repeatedly remarked, the main feature 

 that runs through the whole world of mammals is 

 the concentration of strength upon a shortened 

 jaw, at the cost of the disappearance of teeth. This 

 is most evident in the case of true Carnivora, where, 

 however, the milk-teeth still play an important part. 



10. THE INSECTIVORA, OR INSECT-EATERS. RODENTIA, 



OR RODENTS. CHEIROPTERA, OR BATS. 



Of these three orders the Insect-eaters have 

 already been mentioned from time to time. They 



