THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 49 



living on the earth in which both the dorsal arthropod alimentary 

 canal and the ventral vertebrate one should simultaneously exist in 

 a functional condition ; still it seems to me that even if Ceratodus, 

 Lepidosiren, and Protopterus had ceased to exist on the earth, yet 

 the facts of comparative anatomy, together with our conception of 

 evolution as portrayed in the theory of natural selection, would have 

 forced us to conclude rightly that the amphibian stage in the evolu- 

 tion of the vertebrate phylum was preceded by fishes which possessed 

 simultaneously lungs and gills. 



In the preceding chapter the primitive cartilaginous vertebrate 

 skeleton, as found in Ammoccetes, was shown to correspond in a 

 marvellous manner to the cartilaginous skeleton of Limulus. In a 

 later chapter I will deal with the formation of the cranium from the 

 prosomatic skeleton ; in this chapter it is the mesosomatic skeleton 

 which is of interest, and the consideration of the necessary conse- 

 quences which logically follow upon the supposition that the branchial 

 cartilaginous bars of Limulus are homologous with the branchial 

 basket-work of Ammoccetes. 



Internal Branchial Appendages. 



Seeing that in both cases the cartilaginous bars of Limulus and 

 Ammoccetes are confined to the branchial region, their homology of 

 necessity implies an homology of the two branchial regions, and leads 

 directly to the conclusion that the branchiae of the vertebrate were 

 derived from the branchiae of the arthropod, a conclusion which, 

 according to the generally accepted view of the origin of the respira- 

 tory region in the vertebrate, is extremely difficult to accept ; for the 

 branchial of Limulus and of the Arthropoda in general are part of 

 the mesosomatic appendages, while the branchiae of vertebrates are 

 derived from the anterior part of the alimentary canal. This con- 

 clusion, therefore, implies that the vertebrate lias utilized in the 

 formation of the anterior portion of its new alimentary canal the 

 branchial appendages of the palasostracan ancestor. 



Let us consider dispassionately whether such a suggestion is a priori 

 so impossible as it at first appears. One of the principles of evolution 

 is that any change which is supposed to have taken place in the 

 process of formation of one animal or group of animals from a lower 

 group must be in harmony with changes which are known to have 



