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 vertebre, 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 vertebrze would 
be larger, while the individual vertebrze 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. 
