THE TAIL 



As far as I know there is no reason for believing that any odontocete 

 or mysticete whose remains have yet been discovered had flukes very 

 different from what we are accustomed to consider as characteristic of 

 this order, although it is true that I have not examined the details of the 

 caudal vertebrae of a great many fossils. 



The caudal equipment of the whale has been a subject for the live- 

 liest controversy, as has been the case with so many cetacean details. 

 Thus Gray held the belief that the flukes were derived from the entire 

 hind limbs, while Ryder (1885) considered that they represent the pedes 

 only% The latter s lengthly defense of this belief is a curiously artless 

 combination of established fact and fancy. He argued that the original 

 development in the Cetacea was along somewhat the same lines as that 

 followed by the seals, and that the hind limbs were at one time used in 

 oscillating movements for propulsion. He believed that the feet and tail 

 were later inclosed in a single fold of integument and that finally the 

 bony and muscular part of the hind limbs became atrophied and shrank 

 toward the pelvis, leaving the integumentary part of the pedal expansions 

 attached to the side of the tail. There were some converts to this view, 

 chiefly among those who were unwilling to relinquish the thesis that 

 pinnipeds, sirenians and cetaceans all represent different stages of de- 

 velopment from a common derivative. It seems hardly necessary to 

 point out that if this were the actual situation there would not only be 

 clear and incontrovertible proof of it in cetacean embryos, but if the 

 flukes had been derived from anything except fibrous, dermal dilations 

 of the lateral tail the adult would necessarily exhibit some cartilaginous 

 or muscular relic of the fact in this region. On the contrary I have no 

 fear of contradiction when I say that in the light of present knowledge 

 the evidence is conclusive that the posterior termination of the whale 

 is composed of caudal elements only. 



In attempting to explain the asymmetry of the odontocete skull Stein- 

 mann (1912) made the claim that the Cetacea originated from the ich- 

 thyosaurs, and hence that the whale's flukes have become horizontal from 

 an originally vertical position. This theory is also untenable. Had this 

 been the case the whale would have experienced a period of (say) from 

 one to several millions of years during which its flukes would have been 

 at an angle of 45 degrees (more or less) to both the vertical and hori- 

 zontal. This would have obliged one-half of the erector spinae muscu- 

 lature to have become atrophied and the other half hypertrophied, while 

 a similar fate, but in reverse order, would have overtaken the hypaxial 

 muscles. In truth we would then have a cetacean of astounding asym- 

 metry. 



[199] 



