Mammals to Aquatic Life. Ill 



recognized the points of resemblance as convergent develop- 

 ments of independent origin the structural differences of the 

 two flippers appear much more clearly defined. The contrast 

 between the much greater advance of hyperphalangy in all 

 the toothed whales and its more limited development in the 

 whalebone whales strikes us at once. But there is also a 

 plastic difference to be noticed, in so far as the whalebone 

 whales possess elongated flippers with a straight radial edge, 

 while in the toothed whales the radial edge of the flipper is 

 more or less curved, so that the flipper has acquired a sickle- 

 like form. This difference is not so trivial as it at first sight 

 appears. It has exerted a powerful influence on the skeleton 

 of the hand. In the whalebone whales the flipper appears to 

 be least modified in the smooth whales (the Baltenidse), where 

 we get a rounder form of flipper whose five fingers are all 

 developed with a very small amount of hyperphalangy ; in 

 the fin-whales (the Balsenopteridai) , on the other hand, we find 

 an elongated instead of a rounded flipper, with a straight 

 radial edge, and the consequence of this is the degeneration 

 and disappearance of the thumb. Rudiments of it are still 

 seen in the embryo, which afterwards disappear through fusion. 

 Hyperphalangy has already made a certain advance. In the 

 toothed whales, on the contrary, the finger-rays have adapted 

 themselves to the inflexion of the flipper, and the whole of the 

 five digits are always present. Two different types of flippers 

 are therefore observable in the two groups — the whalebone 

 whales with long extended flippers, the toothed whales with 

 incurved ones. In the former the thumb is lost, in the latter 

 it persists. This is already a highly important difference 

 in the structure of the flipper in the two groups. The 

 disposition of the carpal bones constitutes a further funda- 

 mental difference. We find that the carpus in the adult 

 in many toothed whales and in the embryonic state in 

 many others exhibits an arrangement which is otherwise 

 not characteristic of the Mammalian class ; there are present 

 not only the three proximal but also five distal carpalia, 

 whereas all other mammals only possess four distal carpals ; 

 there is a pisiform and a prsepollex, while the centrale, 

 which in the rest of the Mammalia is only found occa- 

 sionally and in embryonic stages, here often persists and 

 is even found double. The number of the carpal elements 

 thus reaches twelve ; their arrangement is a perfectly typical 

 one, such as must be imagined for the hypothetical, most 

 complicated, and therefore most ancient Mammalian carpus ; 

 and the carpus of the toothed whales displays the greatest 

 agreement with the typical lleptilian carpus, such, for 



