THE EXTERNAL FORM OF WHALES 21 



in any given digit is not greater than what is to be 

 met with among the whales. 



"This," observes Prof. Kiikenthal, "is a case of 

 convergence, of which no better example could be 

 imagined." In two groups of animals so remote 

 in the vertebrate series as are the whales and Ichthyo- 

 saurs we have a modification into a paddle which 

 has proceeded along precisely the same lines, only 

 carried further in the reptile than in the mammal. 

 It will now be interesting to inquire to what degree 

 the limbs of other aquatic animals that have been 

 derived from terrestrial ancestors resemble the fins 

 of the whales. We naturally turn first of all to the 

 Sirenia and to the Seals and Sea-lions. 



In comparing the pectoral limb of the whales with 

 the ichthyopterygium and with the paddle of the 

 Ichthyosaurus, it was unnecessary to point out the 

 absence of nails upon the former ; for the presence 

 or absence of these structures does not bear upon the 

 question of comparison in those two cases. But the 

 absence of nails must be mentioned in comparing the 

 whale's flippers with the limbs of Manatees and Sea- 

 lions ; for the more perfect adaptation of the whales 

 to an aquatic existence has led to the total disappear- 

 ance in the adult of all traces of nails upon the 

 digits. But Dr. Kiikenthal has found rudiments of 

 these structures in the foetus, as has also Leboucq. 

 These structures consist of a thickening of the epi- 

 dermis, which is situated above the last phalanx. 



Now in the Sea-lions and Seals nails are fairly well 

 developed ; but they do not lie at the extremities 



