484 
PRESIDENT’S address. 
in tropical regions, and Pelseneer has pointed out that the 
region of greatest penetration, namely, Indo-China, Bay of 
Bengal, China and Indo-Malay islands, is surrounded by sea 
which is the least salt in the world. To this advantage, rather 
than to equability of climate, he attributes the successful 
colonisation. It seems to me, however, that, having regard to 
the fact that there is a graded salinity at the mouth of every 
river, the low salinity of the tropical seas cannot have so great 
an influence. Probably rivers are effective highroads in pro- 
portion to their size, volume, and the equability of the climate 
of the place. That they have played the largest part in 
colonisation of fresh water there can be no doubt. 
Conclusion. 
In the present conditions of the world the marine fauna is 
separated from that of fresh water by barriers which make 
immigration extremely difficult. The profound difference in 
chemical composition of the water makes transference from the 
one to the other usually immediately fatal, and there are, in 
addition, differences of density, of temperature, and of currents, 
which bar the way. And yet all these obstacles have been and 
can be surmounted. In the Laboratory species have been 
gradually acclimatised to completely altered conditions, and 
Nature herself has performed experiments in acclimatisation on 
a vast scale. But in spite of the magnitude and duration of 
such natural experiments the results seem inadequate and 
subject to no definite law, and the fauna of the Relict lakes of 
the world shows that the isolation of a marine fauna does not 
lead to any great accession to the fresh water fauna. Even 
where the transition from salt water to fresh is most gradual, 
and all conditions for immigration seem to be most favourable, 
the limit is but rarely overstepped, and we have abundant 
evidence of marine faunas dying out in face of slowly lessening 
salinity. 
It seems that the successful adaptation of a species to fresh 
water depends essentially on a physiological variation of the 
organism, without which the most favourable external conditions 
