The Geographical Distribution of Fishes gl 
the facts of present distribution as the result of conditions in 
the past, thus correlating our present knowledge with the past 
relations of land and water as shown through paleontology. 
Dr. A. E. Ortmann well observes that ‘‘Any division of the 
earth’s surface into zoogeographical regions which starts 
exclusively from the present distribution of animals without 
considering its origin must always be unsatisfactory.” We 
must therefore consider the coast-lines and barriers of Tertiary 
and earlier times as well as those of to-day to understand the 
present distribution of fishes. 
General Laws of Distribution.—The general laws governing 
the distribution of all animals are reducible to three very simple 
propositions. 
Each species of animal is found in every part of the earth 
having conditions suitable for its maintenance, unless 
(a) Its individuals have been unable to reach this region 
through barriers of some sort; or, 
(b) Having reached it, the species is unable to maintain 
itself, through lack of capacity for adaptation, through severity 
of competition with other forms, or through destructive condi- 
tions of environment; or else, 
(c) Having entered and maintained itself, it has become so 
altered in the process of adaptation as to become a species dis- 
tinct from the original type. 
Species Absent through Barriers.—The absence from the Jap- 
anese fauna of most European or American species comes under 
the first head. The pike has never reached the Japanese lakes, 
though the shade of the-lotus leaf in the many clear ponds 
would suit its habits exactly. The grunt* and porgies{ of 
our West Indian waters have failed to cross the ocean and there- 
fore have no descendants in Europe or Asia. 
Species Absent through Failure to Maintain Foothold. — Of 
species under (b), those who have crossed the seas and not found 
lodgement, we have, in the nature of things, no record. Of the 
existence of multitudes of estrays we have abundant evidence. 
In the Gulf Stream off Cape Cod are every year taken many 
young fishes belonging to species at home in the Bahamas and 
which find no permanent place in the New England fauna. In 
* Hemulon. + Calamus. 
