GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 15 
they can scarcely be treated singly, and their influences are 
therefore discussed together ; but the laws which govern the 
distribution of animals in the ocean are so different from those 
. which govern the distribution of land and fresh-water spe- 
cies, that they are best treated separately. 
The influence of topography in limiting the diffusion of 
marine species is too evident to require much explanation, 
and yet, uncombined with the influence of temperature, it 
would have little effect ; for it is hardly possible to imagine a 
limit to the migration of species along coast lines and around 
capes from ocean to ocean, were the temperature of the 
water perfectly uniform. Still the mere separation of coasts 
by long intervals of deep water seems to have a direct influ- 
ence in preventing the migration of certain groups of species ; 
as, in the Pacific Ocean, under the same lines of tempera- 
ture,* there are many species, especially of fishes and polyps, 
which are peculiar to each of the great groups of islands. 
The influence of temperature has long been recognized as 
a most powerful cause in limiting the diffusion of marine 
species. Animals, with very few exceptions, are adapted 
for life and reproduction only within fixed limits of temper- 
ature, and a rise above or a fall below these limits, quickly 
puts an end to their existence. Such limits of temperature 
act as a continual check upon the effects of ocean currents in 
transporting species from place to place. Thus the Gulf 
Stream, flowing from the warm coral reefs of Florida and the 
Bahamas, must bear myriads of life-germs to the Bermudas 
and on across the Atlantic toward the Azores; but the iso- 
erymal line of 68° F., which limits, on both sides of the 
equator, the reef-building coralst and most of the tropical 




aoe fact ‘should not be looked that th isot} landi ymal lines indi- 
d as th very little known of deep ocean tem- 
, that it is quite possible that some ‘specie s are retarded from descending to 
a wuiitetent depth to pass from place to Se by the decrease in temperature; still the 
number of species must be small that c masat even with the same temperature, at 
‘very different depths. (Isothermal is u p equa al annual temperature; iso- 
z T ai " + F; hk TA + +h fth 

"} Dana, United States Exploring Expedition, Vol. I, Zoöphytes. 
