i68 



HISTORY OF THE HUMAN BODY 



paratively abrupt one, characterized by a rapid adjustment to 

 the needs of a terrestrial life. It thus follows that the transition 

 forms themselves must have been put at a disadvantage when 

 in competition both with their immediate descendants, which 

 were better fitted for the land, and with their immediate ances- 

 tors, which had never left the water, and their rapid extinction 

 was a necessary consequence. The deficiency in the historical 

 record at this place, however, has not prevented speculation on 

 this subject; on the contrary, it has proved an especially at- 



FIG. 43. Diagrams illustrating the development of the fin skeleton; 

 based on that of selachians. [After WIEDERSHEIM.] In (a) and (c) 

 the right side shows a slightly older stage than the left. 



tractive field for the anatomical philosopher, and the discus- 

 sion of some of the leading theories of this question will be 

 considered farther on in the present chapter. 



Aside from this problem, however, the history of the de- 

 velopment of the limbs is by no means clear in other respects, 

 and although the faith in their complete homology throughout 

 is universal, the manner of their development and the relation 

 of the various forms to one another cannot be agreed upon. 

 Embryology, which is usually so suggestive, is practically 

 silent here, since the record seems in all cases to be much 

 abbreviated. The best that can be done, therefore, is to 

 arrange a sequence of adult forms which seem to show 

 transitions from one type to another, paying as much regard 

 as possible to the lines of descent as indicated by the other 

 parts. In this way have been sketched the histories which 

 follow, and in reading this it must be remembered that a his- 

 tory founded merely on a succession of adult forms, and re- 



