6 
nean, at the same season, to spawn, following the line of the 
European continent when it arrives, and on its return after 
spawning, taking the coast of Africa and passing on through 
the Straits of Gibraltar to the Atlantic. 
Now it will be easy to explain the diminution of this fish 
since historic times, by the fact that of the great quantities 
which enter the Mediterranean to spawn some are caught 
by the fishermen after spawning, but the great majority 
before fulfilling this function. How then can we conclude 
that the cause of this diminution is not owing to the action 
of man? 
It is quite true that many centuries may elapse before 
the complete extinction of a migratory species from the 
vast expanse of the main; perhaps before this exhaustion 
takes place some of those abnormal conditions may occur 
in the history of our planet which will disturb the essential 
conditions of the history of those species, but even in this re- 
mote contingency we must not lose sight of the fact that the 
biological regions are not unlimited, and that every species 
seeks out its abode where nature and its kindred elements 
have produced the required conditions, some species being 
only able to live in low temperature, others in high ones, 
some again cannot live in estuaries, and those species which 
are limited by temperature are again subdivided into others 
separated by the pressure of the waters, some being obliged 
to live at a great depth compared with others, and among 
other limitations there is the question of food and shelter, 
the appropriate motion of the currents ; and lastly, that the 
depletion be not carried to such an extent as to prejudice 
food-supply, and thus be the first step towards an end of the 
industry. 
It is now time to enter on the second head of the theory 
propounded. We have said that the depletion of the 
