Mammals to Aquatic Life. 165 



found in embryos of the latter seem to me not wholly free 

 from doubt. 



In the land-mammals the various divisions of the fore limb 

 have different functions, to which they are adapted ; the struc- 

 ture of these divisions is consequently not the same, but 

 rather each is adapted to its own particular fimction. Now 

 as the fore limb comes to be used more and more as a fin the 

 degree of differentiation of the various parts diminishes, their 

 functions are more nearly the same, and the consequence of 

 this will be that skeleton and musculature will both be influ- 

 enced thereby. As a matter of fact we can trace the loss of 

 differentiation between the various skeletal parts of the hand 

 in the series of aquatic mammals, while at the same time 

 there sets in a gradual reduction of the joints which bind 

 these parts together. The changes therefore which the ante- 

 rior extremity has to undergo in the process of being trans- 

 formed into a flipper rest on purely mechanical grounds. On 

 similar grounds it appears more advantageous for the long 

 phalanges to become somewhat more flexible. Now how is 

 this brought about ? 



We all know that the ossification of the finger-joints takes 

 place in such a way that each skeletal element is preformed 

 in cartilage, in the middle of which there subsequently appears 

 a bony centre, the diaphysis. The ossification of one of the 

 cartilaginous ends now proceeds from this diaphysis, while 

 the other, the epiphysis, receives a separate osseous germ of 

 its own, which does not unite with the diaphysis until later. 

 Now in order to produce more jiexihle elements an incomplete 

 ossification takes place in the aquatic mammals ; a retardation 

 of the process sets in. This retarded ossification shows itself 

 primarily in the diminished size of the diaphysis and the 

 increased size of the epiphysis, while the latter unites with the 

 former either only incompletely or else not at all. Of this we 

 have a whole series of instances among the aquatic mammals. 

 At the same time, however, a retardation also sets in in the 

 ossification of the other end of the joint; the formation of the 

 diaphysis is already to a certain degree completed, while this 

 end still remains cartilaginous ; finally a separate osseous 

 germ will appear in it, and so we have the formation of double 

 epiphyses. We find indications of these double epiphyses in 

 the metacarpals of Plati/pus ; we find them further advanced 

 in the hands of seals and Sirenians, and fully developed in 

 tlie whalebone and toothed whales. 



That it actually is an instance of adaptation to the aquatic 

 life with which we are dealing, is proved by the fact that it 

 is only in aquatic mammals that double epiphyses are found. 



