240 The Geographical Distribution of Fishes 



i. That extinction which comes from modification or pro- 

 gressive evolution, a relegation to the past as the result of a 

 transmutation into more advanced forms. 2. Extinction from 

 changes of physical environment which outrun the powers of 

 adaptation. 3. The extinction which results from competition. 

 4. The extinction from extreme specialization and limitation 

 to special conditions the loss of which means extinction. 5. 

 Extinction as a result of exhaustion. As an illustration of No. i, 

 we may take almost any species which has a cognate species on 

 the further side of some barrier or in the tertiary seas. Thus 

 the trout of the Twin Lakes in Colorado has acquired its present 

 characters in the place of those brought into the lake by its actual 

 ancestors. No. 2 is illustrated by the disappearance of East 

 Indian types (Zanclus, Platax, Toxotes, etc.) in Italy at the end of 

 the Eocene, perhaps for climatic reasons. Extinction through 

 competition is shown in the gradual disappearance of the Sacra- 

 mento perch (Archoplitis interruptus] after the invasion of the 

 river by catfish and carp. From extreme specializaion certain 

 forms have doubtless disappeared, but no certain case of this 

 kind has been pointed out among fishes, unless this be the 

 cause of the disappearance of the Devonian mailed Ostracophores 

 and Arthrodires. It is not likely that any group of fishes 

 has perished through exhaustion of the stock of vigor. 



Barriers Checking Movement of Marine Fishes. The limits 

 of the distribution of individual species or genera must be 

 found in some sort of barrier, past or present. The chief bar- 

 riers which limit marine fishes are the presence of land, the 

 presence of great oceans, the differences of temperature arising 

 from differences in latitude, the nature of the sea bottom, and 

 the direction of oceanic currents. That which is a barrier to 

 one species may be an agent in distribution to another. The 

 common shore fishes would perish in deep waters almost as surely 

 as on land, while the open Pacific is a broad highway to the 

 albacore or the swordfish. 



Again, that which is a barrier to rapid distribution may be- 

 come an agent in the. slow extension of the range of a species. 

 The great continent of Asia is undoubtedly one of the greatest 

 of barriers to the wide movement of species of fish, yet its long 

 shore-line enables species to creep, as it were, from bay to bay, 



