SOME INTERNAL STRUCTURES 47 



The question now becomes pressing, Is this 

 shortening of the sternum a character of whales 



o 



unconnected with anything in particular, or is it 

 related to the aquatic life ? The answer to this 

 question is to be derived from two sources. We 

 have first the argument from analogy. We can 

 consider how far, if at all, the same kind of change 

 has gone on in other aquatic creatures. The Seals 

 and Sea-lions do not help us in the very least ; but 

 then it must be borne in mind that they are com- 

 paratively recent inhabitants of the water. The 

 Sirenia, on the other, offer us a precisely similar series 

 of stages. The " Morskaia korova," " Steller's sea- 

 cow," or Rhytina gigas, had five pairs of ribs reach- 

 ing the sternum ; the Dugong of eastern seas but 

 four ; while in the Manatee the ribs are reduced to 

 three pairs. The sternum, too, in these animals is 

 naturally reduced in correspondence with the failing- 

 attachment of the ribs. But it is somewhat contra- 

 dictory to bear in mind that the first two genera, the 

 least modified as regards ribs, have a crescentric 

 tail more like that of whales, while in other particulars 

 referred to on other pages Rhytina is more whale- 

 like than either of its congeners. To go to quite 

 another group to which we have often had occasion 

 to refer in dwelling upon the peculiarities of whales 

 the Ichthyosaurians were devoid of a sternum, at 

 least of an ossified one ; and the same statement 

 holds good for the Plesiosaurs. There would seem 

 therefore to be some connection between the aquatic 

 life and an absent or rudimentary sternum. 



