364 Evolution and Distribution of Fishes 



The former consists of a compact set of genera and 

 species that are met with over the eastern half of the United 

 States. The interest attaching to them lies in the fact that, 

 following the universal law of action and reaction between 

 environment and organism, while in Chologaster the eyes 

 are well or fairly developed, the body is lithe and plump, 

 and the skin is finely colored, the allied genera Amhlyopsis 

 and Typhlichthys have migrated from the light into the 

 darkness of the extensive caves over the east-central States. 

 As light stimulated and evolved optic organization, with all 

 its associated neuro-muscular connections, so also has its 

 removal caused, in the progressively degenerating types 

 Chologaster agassizi, Amhlyopsis, Typhlichthys, and 

 Troglichthys, progressive absorption of the eyes and their 

 related mechanisms, as well as blanching of the skin-color, 

 till in the last it almost reaches to the point of complete 

 absorption. 



The Percopsidae is regarded by the writer as a family 

 of exceptional importance phylogenetically. This view has 

 already been set forth by skilled ichthyologists. Thus Boul- 

 enger (2 : 620) writes : "This is a most interesting group of 

 fishes, from the resemblance which they bear to Perches, 

 and they have therefore been raised to the rank of a sub- 

 order, Salmopercae, by Jordan and Evermann, who regard 

 them as 'archaic fishes, relics of some earlier fauna, and 

 apparently derived directly from the extinct transitional 

 forms through which the Haplomi and Acanthopteri have 

 descended from allies of the Isospondyli (Malacopterygii) . 

 On the other hand an analysis of their characters shows 

 them to belong to the Haplomi, of which they may be re- 

 garded as highly specialized members, having evolved in 

 the direction of the Acanthopterygii." 



Percopsis and Columbia, each with a single species, are 

 found from North-central Canada westward to the Colum- 

 bia river, and southward to Kansas and Ohio. Such distri- 

 bution suggests that the great set of lakes of late Upper 

 Cretaceous, Eocene and Miocene age, referred to by Cope 

 and many others since, and which occupied a large part of 

 west-central N. America, were not only extremely rich in 



