CONCLUSIONS 479 



7, Hexanchus and Pliotrema have 6, and the remaining Selachi 

 have 5 gill-slits and branchial arches on each side. Now the 

 remarkable thing is that the last branchial arch has a typical 

 structure whether it be the 7th, 6th, or 5th. Its peculiarity 

 consists in the fact that its pharyngobranchial element is 

 attached to that of the preceding arch, and it receives a portion 

 of the trapezius muscle. The function of the last branchial 

 arch is to anchor the branchial basket on to the shoulder- 

 girdle. This being so, in the course of the evolution of forms 

 with 5 branchial arches from forms in which there were 6,* 

 it is impossible to imagine that the transformation took place 

 gradually by reduction from behind ; for if this had occurred, 

 there would have been stages in which the " old last branchial " 

 arch had partly disappeared and the " new last branchial " 

 arch had partially been modified to replace the former, and it is 

 difficult to see how such an arrangement could have fulfilled 

 the function of providing attachment between the branchial 

 basket and the shoulder-girdle. This case is therefore different 

 from that of the limbs, for in the latter there is nothing to 

 prevent gradual transposition of the limb by means of partial 

 modification of adjacent segments. Still less can it be imagined 

 that the number of branchial arches has been altered by 

 reduction from in front because the first two visceral arches, 

 the mandibular and hyoid, are constant throughout the Gnatho- 

 stomes. The only explanation left is that there has been a 

 sudden change in evolution, and that a formwith,say,6 branchial 

 arches gave rise to offspring with 5, without any intermediate 

 stage of functional inefficiency. This conclusion is, of course, 

 interesting from the point of view of evolution, but it is also 

 not without importance as regards the relation of metameric 

 segmentation to differentiation during development. The 

 only difference between this hypothetical offspring with 5, 

 and its parent with 6 branchial arches, is that the raw material 

 for the production of the branchial arches has in the one case 

 been divided up between 5 segments and in the other between 

 6 segments, during development. It is this raw material which 

 is homologous in the two forms, regardless of the numerical 



* Or vice versa. 



