238 The Geographical Distribution of Fishes 



the facts of present distribution as the resuh 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 weU observes that "x\ny 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 earUer 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 t 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 (6), 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 



* nonunion. ■\ Calamus. 



