94 The Geographical Distribution of Fishes 
1. 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. 1, 
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, 
