THE ORIGIN OP VERTEBRATE LIMBS 423 



and most anterior buds which enter the fins have to reach out 

 of position to accomplish it. 



(3) The fact that in the pelvic fin these most anterior muscle- 

 buds are the earliest to appear is not to be taken as a proof 

 that they are phylogenetically older than those which appear 

 later, as the gill-arch theorists assume (Braus '98), but, rather, 

 they appear first because they are most anterior, for it is in the 

 nature of all such serial structures to develop from anterior to 

 posterior, e. g., gills, somites, pronephric tubules, etc.). This 

 condition is observed in the unpaired fins as well as in the 

 paired, for in the dorsals of Cestracion the most anterior buds 

 are the first to develop. The same principle, according to my 

 observations, holds also among the various fins, the pectoral 

 preceding the pelvic, the first dorsal preceding the second, in 

 time of development, etc., yet this cannot be considered a proof 

 that the first dorsal is phylogenetically older than the second 

 nor the pectoral older than the pelvic. (According to the gill- 

 arch theory the pelvic should be the older, yet it develops 

 later than the pectoral.) 



These facts are readily interpreted on the hypothesis that 

 the bases of the fins once extended over a larger number of body 

 segments than at present. In the pelvic it is evident that the 

 shortening has been much greater at the anterior than at the 

 posterior edge of the fin. As a result the present fin is now 

 situated in advance of what was once its posterior limit, though 

 the anterior edge of the present fin is much farther back than 

 formerly. Certainly this is not migration, but a concentration 

 of the fin basis in a manner similar to that occurring in the 

 unpaired fins. 



C. If the six objections given under this heading can be 

 answered, the paired fins should be compared rather than con- 

 trasted with the unpaired fins. 



I. If it were true, as the opponents of the fin-fold theory main- 

 tain, that the skeleton of the unpaired fins consists of modified 

 spinous and hsemal processes, then evidently the skeleton of the 

 paired fins could not have a similar origin to that of the unpaired. 

 Opposed to such a view, however, are the following facts: 



(1) In the first and second dorsals, the anal, and the superior 



