54 THE GENESIS OF SPECIES. [Chap. 



beginning of sucli \iseful development ? There are indeed 

 certain animals of exclusively aquatic habits (the dugong 

 and manatee) which also possess more or less horn on the 

 palate, and at iirst sight this might be taken as a mitiga- 

 tion of the difliculty ; but it is not so, and the fact does 

 not help us one Step further along the road : for, in tlie 

 Iirst place, these latter animals differ so importantly in 

 structure from whales and porpoises that they form an al- 

 together distinct order, and cannot be thought to approxi- 

 mate to the whale's j^rogenitors. They are vegetarians, the 

 whales feed on animals ; the former never have the ribs ar- 

 ticulated in the mode in wdiich they are in some of the lat- 

 ter ; the former have pectoral mammiE, and tlie latter are 

 provided with two inguinal mammary glands, and have the 

 nostrils enlarged into blowers, which the former have not. 

 The former thus constitute the order Sirenia, while the lat- 

 ter belong to the Cetacea. In the second place, the horny 

 matter on the palates of the dugong and manatee has 

 not, even initially, that " strainer " action which is the 

 characteristic function of the Cetacean " baleen." 



There is another very curious structure, the origin or 

 the disappearance of which it seems impossible to account 

 for on the hypothesis of minute indefinite variations. It- is 

 that of the mouth of the 3'oung kangaroo. In all mam- 

 mals, as in ourselves, the air-passage from the lungs opens 

 in the floor of the mouth behind the tongue, and in front 

 of the opening of the gullet, so that each particle of food 

 as it is swallowed passes over the opening, but is prevented 

 from falling into it (and thus causing death from choking) 

 by the action of a small cartilaginous shield (the epiglottis), 

 which at the right moment bends back and protects the ori- 

 fice. Now the kangaroo is born in such an exceedingly 

 imperfect and undeveloped condition, that it is quite unable 

 to suck. The mother, therefore, places the minute blind 

 and naked young upon the nipple, and then injects milk 



