NO. 5 TELESCOPING OF THE CETACEAN SKULL 39 



departed widely from this primitive type ; but in neither is there any 

 telescoping : the bones unite with each other in the normal way. Both 

 zeuglodonts and sea-cows are contrasted with modern cetacea in three 

 peculiarities which must be directly related to the mechanical pres- 

 sures which are brought to bear on the skull. These are: (a) Smaller 

 size of the head in proportion to that of the body, (b) greater length 

 of the neck in relation to that of the head, and (c) distinctly less 

 enlargement of the vertebral processes which serve as areas of attach- 

 ment for the muscles that operate the caudal propellor. It seems 

 obvious therefore that the head, in these aquatic mammals with 

 non-telescoped skulls, has been subjected to a less violent conflict with 

 the water than that of the modern cetaceans, because (a) its area 

 opposed to the water in the act of swimming is less, (b) it is not 

 held so stiffly and uniformly pressed into the resisting element, and 

 (c) it is driven against the water at a lower speed. This last factor 

 is probably the most important of the three. The resistance which 

 water presents to a moving body increases as the square of the 

 velocity. Hence if a porpoise swims twice or three times as fast 

 as a sea-cow its head will be subjected to from four to nine times the 

 backward pressure encountered by the head of the less rapidly moving 

 animal. 



Cetaceans, as is well known, are born in the water. The young 

 must therefore acquire the ability to swim rapidly at an early age. 

 Exactly how soon they gain the power of freely accompanying the 

 adults cannot be stated with any degree of certainty ; but there can be 

 no doubt that the habit of rapid swimming is established long before 

 the skull has reached its full growth. The peculiar behavior of the 

 modern cetacean skull may therefore, as it seems, not impossibly be 

 connected with this faci : that during much of that critical period in 

 which the skull of an ordinary land mammal is rapidly and, so to 

 speak, peacefully accomplishing its process of loose-jointed growth 

 the skull of a young cetacean is fighting its way to adult stature 

 against the two opposing forces of body push from behind and 

 water resistance from in front. 



It may be possible to form a better understanding of the subject 

 of behavior when a considerable number of cetacean embryos repre- 

 senting the earliest stages of growth of the principal bones of the 

 skull can be examined. Unfortunately I have not had access to such 

 material, and I have found very scant published information which 

 bears directly on the problem in question. As regards the odontocetes 

 there appears to be little or nothing: on record concerning the stages 



