Mammals to Aquatic Life. 175 



principle of the formation of small skeletal parts, the other 

 comes into action in aquatic mammals, which use their fore 

 limbs no longer for rowing, but merely for steering and 

 balancing purposes. For the latter function a long fore limb 

 is not only unnecessary but even actually in the way ; and so 

 a shortening of the limb will take place, which will commence 

 at the distal end. Practical observations are in entire accord- 

 ance with this ; the whole of the Cetacean fingers examined 

 for the purpose show that the diminution in the number of 

 phalanges which takes place in the course of individual deve- 

 lopment arises from the fusion of the small terminal 

 phalanges. 



This fusion of terminal phalanges is also found in other 

 aquatic mammals ; thus I was able to determine its presence 

 as a variation in Manatus senegalensis^ in which it affected the 

 third and fourth fingers. 



Now how are we to explain the origin of hyperphalangy 

 phylogenetically ? 



As the fore limb of the terrestrial ancestors of the toothed 

 as well as the whalebone whales developed more and more into 

 the flipper, the skeleton was also affected thereby, in that a 

 retardation of ossification set in. In consequence of this 

 retardation there arose the formation of double epiphyses, 

 which attained to the size of the diaphysis, while their bony 

 nuclei remained separate. The functions of the finger-joints 

 became more and more alike, in that they had to relinquish 

 all their differences of action and to become modified into mere 

 supporting organs ; and this similarity likewise extended to 

 the diaphysis and epiphysis of each finger-joint ; they, too, had 

 only to undertake a supporting function. Now the office of 

 the flippers of the whale as rudders entails the avoidance of 

 large bones ; and this necessity was met first by retarding the 

 ossification of the dia))hyses, and making the two epiphyses 

 equivalent to it, and subsequently by separating the latter 

 from the former ; so that in the place of the single skeletal 

 element there develop three similar smaller ones. Where will 

 this process first come into action ? Clearly where the retar- 

 dation in the ossification is most pronounced, and this is the 

 case in the terminal phalanx. The first secondary phalanges 

 will be developed in the terminal phalanx (as, for instance, in 

 the case of Manutus and Ilulicure) ; the further advance of 

 the process affects the other finger-joints also, and finally 

 begins to take place in the bones of the forearm of certain 

 whales. Simultaneously with this process the several 

 division-products grew to resemble one another, owing to 

 their having similar functions to perform j the morphological 



