Variations in the Position of the Pectoral Fin. 321 



and 39 per cent, in the last. Judging by these maxima, 

 therefore, tiiere is a total translocation of 16 per cent. o£ the 

 length of the body. It is interesting to note tiiat if the fifty 

 smallest sticklebacks are taken the maximum falls upon the 

 22 per cent. line. 



Fie 



30 3 



Figr. 3. 



Fi-. 4. 



The immediate cause of the shifting of the fin is to be 

 found in the fact that the coraco-scapular plate is at first a 

 very insignificant portion of the pectoral skeleton, but as 

 development advances it broadens out antero-posteriorly. 



The ultimate cause may be the change in function of the 

 pelvic fin from that of a true fin to that of a mere organ for 

 attack or defence. The pectoral fin has therefore to perform 

 the combined functions of the two pairs. 



In view of the phylogenetic shifting forwards of the pelvic 

 fin in Teleosts, it would be instructive if some worker with a 

 large series of types at his disposal would ascertain if there 

 is a corresponding phylogenetic shifting back of the pectoral 

 fin. At present I can find no explanation of the fact that 

 the fin shifts most rapidly just at those points in the ontogeny 

 where my collection of specimens is poorest. 



A further study of figs. 2-4 suggests that the range of 

 variation in the position of this fin increases as life advances. 

 In the first group the range is 8 per cent., in the second 9 

 per cent., in the third 13 per cent. 



