348 THE POPULAR SCIENCE MONTHLY. 



tance of the individual vertebrae are simply part of this work of 

 making a better fish. Not a better fish for man's purposes for 

 Nature does not care a straw for man's purposes but a better 

 fish for the purposes of a fish. The competition in the struggle 

 for existence is the essential cause of the change. In the center 

 of competition no species can afford to be handicapped by a weak 

 backbone and redundant vertebree. Those who are thus weighted 

 can not hold their own. They must change or perish. 



The influence of cold, darkness, monotony, and isolation is to 

 limit the struggle for existence, and therefore to prevent its 

 changes, preserving through the conservation of heredity the 

 more remote ancestral conditions, even though they carry with 

 them disadvantages and deficiencies. The conditions most favor- 

 able to fish life are among the rocks and reefs of the tropical seas. 

 About the coral reefs is the center of fish competition. A coral 

 archipelago is the Paris of fishes. In such regions is the greatest 

 variety of surroundings, and therefore the greatest number of 

 possible adjustments. The struggle is between fish and fish, not 

 between fishes and hard conditions of life. No form is excluded 

 from the competition. Cold, darkness, and foul water do not 

 shut out competitors, nor does any evil influence sap the strength. 

 The heat of the tropics does not make the water hot. It is never 

 sultry nor laden with malaria. The influence of tropical heat on 

 land animals is often to destroy vitality and check self-activity. 

 It is not so in the sea. 



From conditions otherwise favorable in arctic regions the 

 majority of competitors are excluded by their inability to bear 

 the cold. River life is life in isolation. To aquatic animals river 

 life has the same limitations that island life has to the animals of 

 the land. The oceanic islands are behind the continents in the 

 process of evolution. In like manner the rivers are ages behind 

 the seas. 



Therefore the influences which serve as a whole to intensify 

 fish life, and tend to rid the fish of every character or structure it 

 can not "use in its business," are most effective along the shores 

 of the tropics. One phase of this is the reduction in numbers of 

 vertebrae, or, more accurately, the increase of stress on each indi- 

 vidual bone. 



Another phase is the process of cephalization, the process by 

 which the head becomes empnasized and the shoulder bones and 

 other structures become connected with it or subordinated to it. 

 Still another is the reduction and change of the swim-bladder and 

 its utter loss of the function of lung or breathing organ which it 

 occupied in the ganoid ancestors of modern fishes. 



Conversely, as these changes are still in operation, we should 

 find that in cold waters, deep waters, dark waters, fresh waters, 



