II.] INCIPIENT STRUCTURES. 47 



have pectoral mammae, and the 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 latter belong to the 

 Cetacea. In the second place, the horny matter on the 



palates of the clugong 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 young 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 

 orifice. 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 into it by means of a special muscular envelope of 



