104 THE AMERICAN NATURALIST [Vol. XLI 



groups. But this explanation will not suffice for other cases, 

 hence the probability that it is true for none. 



In the frog as in all Ichthyopsida, there are but ten cranial 

 nerves, while in the mammals there are twelve. There is no 

 doubt that as far back as the tenth the nerves are exactly homol- 

 ogous in Amphibia and in the mammals. Relations to brain and 

 to points of distribution place this beyond question, but what 

 shall be said of the mammalian eleventh and twelfth? Are both 

 of these nerves from the post-cranial region which have been 

 transferred to the skull ? If so, does it not follow that the cranium 

 in the higher vertebrates is not the exact equivalent of that in the 

 lower? and that the differences have been brought about by the 

 transformation of cervical into occipital vertebr{3e. If this, in 

 turn, be so, are the occipital bones of the frog homologous with 

 those of the mammal ? Or are the basi-, ex- and supra-occipitals 

 of the one merely analogous of those of the other? Is Huxley's 

 argument for the derivation of the mammals from the Amphibia 

 because of the double occipital condyles in the two groups based 

 upon analogies rather than on true homologies ? Are the condyles 

 in Amphibia and Mammals not homologous but rather homo- 

 plastic formations? 



Carrying this matter further back in the body, how are we to 

 explain that apparent shifting of the pelvis in such a form as 

 Necturus as described by Bumpus, Parker and others? Are 

 somites ten, twenty and the like exactly equivalent in the normal 

 and aberrant forms? And has there been an actual shifting of 

 the pelvic girdle from one somite to the next in some individual ? 

 Or has there been an actual intercalation of vertebrte, the one to 

 which the ilium is attached being constantly the same morpho- 

 logically if not serially? Or, lastly, have the limbs and their 

 arches arisen from a continuous fin fold and has every somite 

 which contributes to that fold the potentiality of limb formation 

 with all that this implies ? 



To take another case. In Amphioxus there are a large number 

 of gill slits, a ninnber which is doubled during development by the 

 formation of the 'tongue bar.' Right behind the last gill slit 

 comes the entrance of the hepatic duct into the alimentary tract, 

 there thus being no oesophagus nor stomach intervening between 



