THE EVIDENCE OF THE RESPIRATORY APPARATUS 149 
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 Ammoceetes. 
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 branchie of the vertebrate were 
derived from the branchize 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 
branchize of Limulus and of the Arthropoda in general are part of 
the mesosomatic appendages, while the branchia of vertebrates are 
derived from the anterior part of the alimentary canal. This con- 
clusion, therefore, implies that the vertebrate has utilized in the 
formation of the anterior portion of its new alimentary canal the 
branchial appendages of the paleostracan 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 
