LAW OF SPECIALIZATION OF PHYLETIC BRANCHES 209 



cavity or femur ; in the Siren or Halicore of the 

 present day the pelvis has almost disappeared and 

 is replaced by a pair of slender rod-shaped bones 

 united by a symphysis and corresponding to the 

 ischion alone. As is the case with the Ungulates, the 

 specialization of the Sirenians has taken place on 

 parallel lines in the different phyletic branches 

 representing the evolution of that order. 



The two foregoing examples show us a functional 

 specialization obtained by a process of reduction of 

 certain organs. At other times, on the contrary, 

 there is an abnormal multiplication of certain 

 elements. Such a case is presented by the special 

 structure of the natatory paddle in the Icthyo- 

 sauridse. Here, the humerus is a short and flat- 

 tened bone ; then come numerous rows of poly- 

 gonal plates almost identical, in which it is some- 

 what difficult to distinguish the radius and the 

 cubitus ; and then two rows of carpal bones, a row of 

 metacarpals, and numerous phalanges formed of 

 bones gradually lessening towards the extremity of 

 the limb. The most remarkable specialization about 

 this natatory paw, covered with one common skin, 

 consists in the indeterminate number of the digits, 

 which may reach eight or nine in certain species. 

 This is a unique peculiarity which greatly struck 

 Haeckel and Gegenbaur and led them to detach the 

 Ichthyosaurs from the other Reptiles and derive 

 them from the Selachians. Baur sees in this, on the 

 contrary, as in the fins of the Cetacea, a simple 

 secondary adaptation to aquatic life and considers 

 the Ichthyosaurs as very specialized descendants 

 of the terrestrial Reptiles, more or less related to the 

 p 



