PRIMITIVE FISHES. 109 



That pait of the animal kingdom with which wo are 

 now concerned has become so narrow, even before we have 

 left the Archilithic Epoch, that but a single one of the 

 seven tribes of the animal kingdom forms the object of our 

 study. Even within this tribe we have passed the lowest 

 steps, and have risen above the Skull-less (A crania) and 

 Double-nostrilled Vertebrates (Monorhina), to the class of 

 Fishes. The latter are the first of the great main division 

 of Vertebrates distinguished by mouths with jaws and by 

 double nostrils (Amphirhina, or Gnathostoma). From Fishes 

 we start again, as from that class of Vertebrates which are 

 indubitably shown by Comparative Anatomy and Ontogeny 

 to be the ancestral class of all higher Vertebrates, all Am- 

 phirhina. Of course no existing Fish can be regarded as 

 the direct parent-form of the higher Vertebrates. But it is 

 equally certain that from a common extinct Fish-like 

 parent-form we may trace all those Vertebrates from Fishes 

 up to Man, which are included under the name of Am- 

 phirhina. If this primseval parent-form were extant, we 

 should undoubtedly describe it as a genuine Fish and class 

 it among Fishes. Fortunately, the Comparative Anatomy 

 and Classification of the Fishes has been so far advanced 

 (thanks to the labours of Johannes Muller and Gegenbaur) 

 that we can very clearly distinguish these most important 

 and interesting genealogical relations. 



In order correctly to understand the human pedigree 

 within the Vertebrate tribe, it is very important to bear in 

 mind the distinguishing characteristics, separating Fishes 

 and all the other Double-nostrils (Amphirhina) from 

 Single-nostrilled and Skull-less Animals (Monorhina and 

 Acrania). These very distinguishing characteristic marks 



