44 THE INFANCY OF ANIMALS 



Now in all living whales, where the jaws are armed with 

 teeth during life, these are simple pegs ; but in certain 

 fossil whales, which are in other respects also less highly 

 specialised than their modern representatives, the teeth 

 bear three cusps. 



Thus, then, the embryonic teeth of these whales recall 

 those of the ancestral zeuglodont. The " whale-bone," 

 then, represents an entirely new structure replacing the 

 teeth : an adaptation to new methods of feeding developed 

 during the struggle for existence. A parallel case is 

 furnished by that quaint-looking relic the Duck-billed 

 Platypus iprnithorhynchus). This animal was long sup- 

 posed to be as innocent of teeth as a whale-bone whale, 

 curious horny pads having taken their place. 



But a few years ago the astonishing discovery was 

 made that for a time the jaws of the young platypus 

 are armed with teeth, answering to the grinders, and 

 these too of a peculiar and complicated pattern found 

 in no other mammals. Speedily, however, the skin of 

 the gums around and under the teeth hardens, until it 

 attains a horny consistency, by which time the roots of 

 these teeth have become absorbed and the crowns are 

 in consequence shed. So that the adult Duck-bill grinds 

 its food by means of horny pads, which seem to serve 

 better than the teeth which they replaced. 



But young mammals furnish us with many other records, 

 no less fleeting and transitory, of the remote past : records 

 which must be sought for sometimes during embryonic, 

 and sometimes during later stages of development, when 

 they vanish without leaving a trace behind them. To take 

 a case in point. The whales, porpoises and dolphins, as 

 everybody knows, have lost every trace of hair from 

 their bodies, save a few bristles which appear on the lips 



