238 HISTORY OF THE HUMAN BODY 



able. It must be noted, however, that this serial homology is 

 clear only in the case of the distal portion of the limb, the part 

 beyond the elbow or knee, while in the portion proximal to this 

 point, there is very little suggestion of such a parallelism. 

 From this may be drawn the following conclusions : Granting 

 that both limbs have arisen from a similar origin, and were 

 alike at the start, it is allowable to suppose that the distal 

 portions, being used in a similar manner, have either retained 

 their primitive structure, or have differentiated alike, up to the 

 point exhibited by the present-day urodeles ; the proximal por- 

 tions, facing from the start radically different problems con- 

 nected with the poise of the body, the varied action of different 

 parts of the trunk, and other differentiating factors, have 

 become modified along different lines, and have attained results 

 that suggest little of the original homology. 



The fin-fold theory of the origin of the limbs, given in 

 the previous chapter, throws but little light upon the theory of 

 serial homology, and, it must be confessed, even stands some- 

 what in the way of such an hypothesis, since, although in its 

 primitive form, the fin-fold may be considered to have been 

 made up of similar elements, repeating themselves metameric- 

 ally, and appearing probably as skeletal rays supplied with 

 muscles from the trunk musculature in the form of " myotomic 

 buds/' yet there is no suggestion that an identical number of 

 these elements was originally taken in the case of the two sets 

 of limbs or that the strictly pentadactylous character of the 

 hand form could have been in any sense primitive, or could 

 have existed at the time at which the two limbs might be sup- 

 posed to have been strictly identical. 



However, as opposed to all theory in the matter, and it must 

 be remembered that the fin-fold theory itself rests upon very 

 little actual evidence, there is the fact of the actual close corre- 

 spondence in the fore and hind limbs of urodeles in general, 

 and especially in the case of Necturus, the particular form 

 which from other reasons is considered especially primitive, 

 perhaps even the oldest living representative of all animals 

 possessing the hand form of appendage. 



Among mammals the limbs of the primates, even in the 



