226 FOOT-NOTES TO EVOLUTION. 



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 activity. It is not so in 

 the sea. 



From conditions otherwise favourable in arctic re- 

 gions 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 far behind the continents in the pro- 

 cess 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 char- 

 acter 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 individual 

 bone. 



Conversely, as the causes of these changes are still 

 in operation, we should find that in cold waters, deep 

 waters, dark waters, fresh waters, inclosed waters, and 

 in the waters of past geological epochs, the process 

 would be less complete, the numbers of vertebras would 

 be larger, while the individual vertebrae remain smaller, 

 less complete, and less perfectly ossified. 



This, in a general way, is precisely what we do 

 find in examining the skeletons of a large variety of 

 fishes. 



