186 W. K. BROOKS. 



Dohrn says (page 60) that these assumptions " set the relation 

 between tlie fishes and the ascidians in the right light/' although 

 the perforation of the gut, which the hypothesis is to explain, is 

 not only left unaccounted for, but is multiplied so many times 

 that, like the man with an unclean spirit, its last state is worse 

 than the first. 



Dohrn says that the secondary nature of the mouth of the verte- 

 brates is proved by its very late appearance in the young vertebrate 

 after its embryonic body and its great systems of organs are fully 

 formed, and by the fact that, when it does make its appearance, it 

 does not lie at the anterior end of the body, in the place which it 

 finally occupies in the great majority of vertebrates, but at a spot 

 some distance behind this place. 



It is not possible to attach much Aveight to either of these argu- 

 ments, for slight changes in the position of organs are not unusual, 

 and it is well known that the ontogenetic acceleration or retarda- 

 tion in the relative time of appearance of structures is by no means 

 exceptional, and it would be as safe to assume that the change in 

 the pitch of the voice of man is phylogenetically older than the 

 sexual maturity of the ancoetors of man, as it is to assume, from 

 the same sort of evidence, that the aortic system of vertebrates is 

 older than the mouth. 



The vertebrate mouth unquestionably bears a great morphological 

 resemblance to a pair of gill-slits. As Dohrn points out, it is bor- 

 dered, like the gill-slits, by a pair of visceral arches, it lies in front 

 of the first pair of true gill-slits, it arises at the same time with 

 them in the embryo, and like them it opens into a section of the gut. 



A ventral view of a shark shows the resemblance between the 

 mouth and the true gill-slits in the most impressive way, and if 

 any pair of them were to be united with each other at their ventral 

 ends, they would become perfectly equivalent to the mouth. The 

 armature of the mouth is repeated on the gills, and there is reason 

 to believe that the jaw-arches have at one time carried gills like the 

 gill-arches. 



This resemblance is not imaginary. Beyond all question it is 

 real, and it is certainly most remarkable and suggestive, but does 

 it prove that the vertebrate mouth is phylogenetically a pair of 

 gill-slits ? 



