EXTERNAL FEATURES 



smooth head can cleave the water with least resistance, and conversely 

 that no whale with a broad, blunt forehead which piles up the water 

 before it can attain to highest speed without expending a disproportion- 

 ate amount of energy. But whales' heads are of almost every conveiv- 

 able shape, varying all the way from those with an excessively long, 

 tweezer-like beak and small head, to the cachalot with its amazing bulk 

 of rostral tissue. In consequence we are forced to believe that in the 

 Cetacea, no matter how strong the stimulus for a stream-line snout and 

 head, the opposing stimulus for the development of an unwieldly frontal 



*wv\ 



FIGURE 10. Heads of porpoises, illustrating frontal prominences and length 

 of rostra: (a) Globiocephala; (b) Phocaena; (c) Tur stops; (d) Delphinus ; 

 (e) the extinct Zarhachis (restoration after Kellogg) ; and (/) Monodon. 



fat organ, or else for a huge and rather blunt snout (as in the Balaeni- 

 dae) has at times proven the more powerful. The inference is there- 

 fore drawn that moderately rapid propulsion through the water for many 

 millions of years need not necessarily bring about a good stream-line 

 form to the head. But it is logical to accept the thesis that the head is 

 just as amenable to stream-line influences as is the remainder of the body, 

 so it may therefore be accepted as a fact that the stimulus that has at 

 times resulted in a cetacean head that offers more resistance to the water 

 than all but a very few of the terrestrial Mammalia, is extremely strong. 



[55] 



