THE FISHES OF JAPAN. 79 



Doubtless, local oscillations in coast lines have taken place and are 

 even in operation at present, but the time has passed when a dance of 

 continents can be invoked to explain anomalies in animal distribution. 

 Most of these will be found to have simple causes, when we know 

 enough of the facts in the case to justify a hypothesis. 



The laws governing animal distribution are reducible to three very 

 simple propositions: 



Every species of animal is found in every part of the earth having 

 conditions fit for its existence, unless 



(a) Its individuals have been unable to reach the region in question 

 through barriers of some sort, or, 



(6) Having, reached the region, the species is unable to maintain itself 

 through lack of capacity for adaptation, through severity of competition with 

 other forms, or through destructive conditions 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 distinct from the parent type. 



In general, the different types of fishes are most specialized along 

 equatorial shores. The processes of change through natural selection 

 take place most rapidly there and produce more far-reaching modifi- 

 cations. The coral reefs of the tropics are the centers of fish-life, 

 corresponding in fish economy to the cities in human affairs. The 

 fresh water, the Arctic waters, the deep sea and the open sea represent 

 ichthyic backwoods — regions where change goes on more slowly and in 

 which archaic types survive. 



The study in detail of the distribution of the fishes of the tropics, 

 is most instructive. The study of the origin of the fish groups of Japan 

 affords a fascinating introduction to its multifarious problems. 



