358 Evolution and Distribution of Fishes 



families a well-developed air-bladder, inherited from ganoid 

 ancestors, is seen In all the freshwater types, and in reduced 

 state usually in marine genera. But in the amphibious 

 Retropinna^ and in the deep-sea Salanx it is absorbed. 



Subsequent to the cataclysmic volcanic activity that 

 prevailed during late Cretaceous or early Eocene time, and 

 which greatly obliterated ganoid and evolving teleost life, 

 the Salmonidae, evidently derived from freshwater clupeold 

 ancestry, seem gradually to have spread as genera of 

 colonial fishes, from the Miocene period onward. They 

 are restricted to the northern hemisphere, which they may 

 be said to encircle. 



Against a possible marine ancestry for Salmonidae many 

 grave objections can be urged. Thus the more primitive 

 genera are wholly or mainly freshwater, the more evolved 

 ones are marine or even deep sea. Again it is difficult to 

 imagine genera like Salmo, Coregonus, and Thymallus, 

 which show few or no truly marine species, becoming dis- 

 persed as they are over land areas of the northern hemi- 

 sphere, if they originally were marine — even coastal — de- 

 rivatives. A considerable number further, show the ana- 

 dromous or "homing" instinct, in that though often mi- 

 grating seaward to feed, they return to rivers or lakes to 

 spawn. And as is known to all, the efforts some show — like 

 the salmon — in overcoming obstacles in order to reach real 

 inland regions, is one of the most arresting facts in natural 

 history. The swim-bladder, also, is highly developed in 

 primitive and freshwater types, but gradually becomes small 

 and even absorbed in marine species. These with other 

 strong reasons compel the writer to accept a freshwater 

 origin for the family. 



Closely related to the last are the Gonorhynchidae and 

 Cromeriidae. The former can be traced back in the genus 

 Char'ttosomus to upper Cretaceous marine beds of Lebanon, 

 and in Notogoneus to the lower or Green River Eocene of 

 western N. America, as well as the upper Eocene and 

 Oligocene of Europe. The only living species — Gonorhyn- 

 chus greyi — is met with along the coastal regions of the 

 eastern hemisphere, and the absence of an air-bladder is 

 noteworthy. The latter family consists only of a single 



