SOME INTERNAL STRUCTURES 55 



diaphragm, the partly tendinous, but chiefly fleshy, 

 septum which separates the chest cavity from that 

 in which are lodged the liver, intestines, and stomach, 

 has a vertical direction, and stands, as it were, upright 

 in the body. 



In the whales, on the other hand, the chest cavity 

 is more barrel-shaped, oval in section sometimes, 

 indeed, transversely oval ; its dorsal boundary is 

 much longer than its ventral, and in consequence 

 the diaphragm is distinctly, and mostly very, oblique 

 in direction. It is, however, one thing to state these 

 differences, and quite another to assert that they are 

 modifications connected with the aquatic habit. It 

 might be suggested, in the first place, that these 

 marks of distinction are merely characteristic of 

 whales, just as it is characteristic of one division 

 of whales to have a free malar bone, a fact which 

 is simply of classificatory significance, and has no 

 bearing (at least so far as we can see) upon any 

 special difference in the mode of life of its possessor. 

 Furthermore, the obliquity of the diaphragm might 

 be associated with the shortening of the sternum, 

 which is so marked a character of the whales, 

 especially of the whalebone whales. A whole series 

 of facts, however, upset these at first sight reasonable 

 objections, and seem to prove the contrary, i.e., that 

 the modifications in question are really connected 

 with the aquatic life, and with nothing else. 



In the otter, and still more in the seal, which are 

 examples of two stages in the literally downward pro- 

 gress of a land animal towards an aquatic existence, 



