AQUATIC MAMMALS 



dus, Megaptera, and Rhachianectes the whole head is either flattened 

 dorso-ventrally or moderately in transverse direction (Rhachianectes). 

 In these the vertebrae are either all free, or, according to Andrews 

 (1916) "two or more of the cervical vertebrae usually become ankylo- 

 sed," although I personally have seldom noted this condition. This 

 lack of complete ankylosis in mysticetes whose heads average larger than 

 in odontocetes may be attributed to the common belief that the former 

 group is not as far advanced along the evolutional road or as "special- 

 ized" (ambiguous term!) as the latter, and consequently that the stimu- 

 lus for fusion has not been operative for as long a time. In balaenid 

 whales (Balaena and Eubalaena), however, although the skull is no 

 longer, the head is of prodigeous depth. This increase in total size and 

 mass of head, necessitated by the hypertrophical development of the 

 baleen equipment, has apparently obliged these whales to adopt a make- 

 shift for the accomplishment of swimming. This is even more the case 

 in Physeter, whose great spermaceti organ has enlarged the head to a 

 degree where the snout projects several feet beyond the rostrum. In these 

 it would seem that if the pivot of motion occupied its more natural posi- 

 tion in the anterior thorax, the increase in the relative size of the head 

 would tend to place this too far to the rear or too near to the lineal cen- 

 ter of the animal for best efficiency. Whereas in the usual fusiform 

 method of swimming at high speed both the tail and head are curved 

 from some point in the anterior thorax, a disproportionately large head 

 would disturb the proper balance, and it is suggested, partly by inference 

 and partly by the shift forward of the greatest circumference of the body, 

 that in such whales the center of motion has, to as large an extent as it 

 was able, migrated from the thorax to the posterior part of the skull. 

 Theoretically this is rendered possible by the increase in the inertia of 

 the head and the essentially homogeneous nature of the dorsal cervical 

 and the dorsal thoracic musculature. But this condition is in principle 

 less efficient and has resulted in the definite reduction of speed. The 

 stimulus for excessive head size has simply proved stronger than any 

 forces acting in antagonism and under the conditions which the animals 

 have encountered it has not been too great a handicap for continued exis- 

 tence. 



Thus the neck, body and tail of these whales seem to all intents to 

 comprise the lever arm for swimming and the head can more properly be 

 compared to a fulcrum (as in 4, figure 1) . It might thus be said that 

 while in porpoises the body wags the head, in the sperm and right whales 

 the tendency is for the head to wag the body. The neck is functionally 



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