164 Dr. W. Kiikenthal on the Adajytation of 



Llances to arise owing to convergence, and to be therefore of 

 no value in tracing phylogenetic affinities. Moreover the 

 differences in the structure of the skin in the case of the 

 whalebone and toothed whales came into much greater promi- 

 nence after the results of convergence had been eliminated. 

 On the ground of these differences alone we are justified in 

 maintaining that the toothed whales are of much earlier origin 

 than the whalebone whales, and that the terrestrial ancestors 

 of the two divisions were not identical ; and with this we 

 arrive at the first justification for the assertion that tlie whales 

 are of diphyletic origin. 



This assertion admits of being proved equally well with the 

 help of other systems of organs. The fore limb appears to be 

 particularly well adapted for this purpose. 



The great difference which we noticed even in the skin of 

 aquatic mammals, according as the particular animals were 

 exclusively water-dwellers or merely amphibious, is intensified 

 in the case of the fore limb. It is not until we reach the 

 animals which pass the whole of their time in water that we 

 find the fore limb developing into a fin. The tendency 

 towards the formation of swimming-membranes between the 

 digits is common to all the water-mammals. In exact ratio 

 to the degree of general adaptation to aquatic life do we find 

 this membrane either just indicated, or uniting the digits, or 

 finally enveloping them so that they are no longer visible 

 from the exterior. 1'he comparative anatomy of the creatures 

 which we shall now proceed to indicate exhibits the progres- 

 sive development of the swimming-membranes. In a certain 

 number of aquatic mammals, such as Arvicola amphibius, 

 Bydrornys chrysogaster^ Fiber zibethicus, and others, no 

 swimming-membranes whatever are to be found ; others, as 

 Hydrochariis, possess rudiments ; others, such as the beaver, 

 have webs on the hind feet only, which then have to perform 

 the bulk of the work in swimming; others again, as Ornitho- 

 rhynchus, Lutra, and Enhydris, have webs reaching to the 

 claws on the fore as well as on the hind feet ; while in the case 

 of others the swimming-membranes are expanded by means 

 of strands of connective tissue which project beneath the 

 terminal phalanges, as in the Pinnipedia, until finally the 

 whole fore limb is enveloped in the swimming-membrane, as 

 in the Sirenians, which still show traces of nails, and in the 

 whalebone and toothed whales, which have lost even these. 

 Leboucq's "^ statements about rudiments of nails having been 



* Fl. Leboiicq, " Reclierclies sur la morphologie de la main cbez les 

 mammileres marins," Archives de Biologic, torn, ix., 1889. 



