LAW OF SPECIALIZATION OF PHYLETIC BRANCHES 207 



organ or a group of organs more or less connected 

 from the functional point of view. Specialization, 

 indeed, most frequently seems to have no other aim 

 than the gradual improvement of some determinate 

 function, such as natation, flight, leaping, running, 

 etc. In this last order of ideas, all naturalists are 

 acquainted with the fine researches of Kowalevsky 

 and of Cope on the progressive transformation of 

 the plantigrade and five-fingered or five-toed, paw of 

 the primitive Ungulates into a semi-plantigrade one, 

 then into a digitigrade, and finally into the un- 

 guligrade, with one or two toes, of many modern 

 Ungulates. This transformation is effected by 



(1) a gradual upward movement of the metapods, 

 whose position gets nearer and nearer to the vertical ; 



(2) a lengthening of these metapods, accompanied 

 by a correlative elongation of the whole limb ; 



(3) an enlargement of certain metapods and digits 

 at the expense of their neighbours, which become 

 reduced and finally disappear ; (4) lateral displace- 

 ment and a more solid setting of the bones of 

 the carpus and of the tarsus, arranged primitively 

 in parallel ranks ; (5, and last) a welding together 

 of several parts at first separate from the carpus, 

 the tarsus, and the metapod. Kowalevsky, Cope, 

 and Osborn have attempted some ingenious mechani- 

 cal explanations of these phenomena of adaptation 

 for running, that is to say, for a specialization pro- 

 duced on parallel lines in the course of ages and at 

 a more or less rapid pace in the most varied branches 

 of the Imparidigitse or the Paradigitse. 



The Sirenians offer us on their side a marvellous 

 example of a better and better specialized adapta- 



