xv PROBABLE ORIGIN 229 



and many extinct forms, not only the Zeuglodons, but also 

 true dolphins, as the Squalodons, having a distinct heterodont 

 dentition, the loss of which, though technically called a 

 " degradation," has been a change in conformity to the habits 

 and needs of the individuals. So much may be considered 

 very nearly, if not quite, within the range of demonstrated 

 facts, but it is in determining the particular group of 

 mammals from which the Cetacea arose that greater difficulties 

 are met with. 



One of the methods by which a land mammal may have 

 been changed into an aquatic one is clearly shown in the 

 stages which still survive among the Carnivora. The seals 

 are obviously modifications of the land Carnivora, the Otarise, 

 or sea-lions and sea-bears, being curiously intermediate. Many 

 naturalists have been tempted to think that the whales 

 represent a still further stage of the same kind of modifica- 

 tion. So firmly has this idea taken root that in most 

 popular works on zoology in which an attempt has been 

 made to trace the pedigree of existing mammals, the Cetacea 

 are definitely placed as offshoots of the Pinnipedia, which in 

 their turn are derived from the Carnivora. But there is to 

 my mind a fatal objection to this view. The seal of course 

 has much in common with the whale, inasmuch as it is a 

 mammal adapted for an aquatic life, but it has been converted 

 to its general fish-like form by the peculiar development of 

 its hind -limbs intdt instruments of propulsion through the 

 water ; for though the thighs and legs are small, the feet are 

 large and are the special organs of locomotion, the tail being 

 quite rudimentary. The two feet applied together form an 

 organ very like the tail of a fish or whale, and functionally 

 representing it, but only functionally, for the time has, I trust, 

 quite gone by when the Cetacea were defined as animals with 

 the " hinder limbs united, forming a forked horizontal tail." 

 In the whales, as we have seen, the hind-limbs are aborted 

 and the tail developed into a powerful swimming organ. Now 

 it is very difficult to suppose that when the hind-limbs had 

 once become so well adapted to a function so essential to the 

 welfare of the animal as that of swimming, they could ever 

 have become reduced and their action transferred to the tail ; 



