SWIMMING
'
ANIMALS
_
89
Zones or in the vicinity of rocky coasts, as opp0sed to those
of more open water.
Such differences can be noticed in
-
comparatively small areas ; but there are, as well, differences
to be found Over very large regions.
‘Everyone
knows that if he visits tropical latitudes he-_will see shes
_
that he never sees in more_ndrthern waters. A visitor to
such widely separated sh—markets as those of Grimsby in
_
England, Trieste in Italy, and Colombo in Ceylon, Will at
once be convinœd of this.
Each market will display its
own characteristic sh, and it is d‘o‘ubtful whether any sh
would
be
found common to all three '
markets.
Such
localization of faunas is
easily understandable on land,
where
barriers as deserts, high mountain ranges or
large areas of water, limit the dierent animals to their own
'
special regions.
But in the sea, with all the oceans of the
world connected, there are no such purely mechanical
barriers present to limit the dispersal of the different kinds
of sh and prevent them from presenting a perfectly uniform
_
distribution from ocean to ocean and sea to sea.
How is it
then that one nds nevertheless that certain widely separated areas each possesstheir own characteristic population
of shes ?
It is evident that there must.be s0me barrier,
-
and a general exploration of the chemical and physical
properties of sea water has shown that in all probability the
chief factor in the distribution of the shes is the temperature
of the water itself;
‘
=
.
.
It is generally to be noticed that in our northern waters, '
wherever the temperature of the water is less
ten
_
degrees Centigrade, our typical northern shes_
will be
found, cod, halibut, haddock—, herring, and many others.
'
On our south—western coasts, those of Devon and_Cornwall,
we nd that we are on the scuthern limit of the distribution
-
'
of these northern shes ; and, at the same time, here occur
.
the northern limits of certain southern warm—water loving