470 
PRESIDENT’S address. 
individual, the range of toleration is usually large, but it is well 
known that one of the first results of changed conditions is dis- 
turbance of reproductive power. Since the conditions in nature 
can rarely allow of changes so gradual as are possible in the 
laboratory, an animal which is to colonise fresh water effec- 
tually must be one which can resist relatively great fluctuations 
of salinity without injury to its powers of reproduction. There 
are certainly great individual, sexual, and specific differences in 
susceptibility to the same stimulus, and without a certain 
innate adaptibility the most favourable circumstances would not 
lead to acclimatisation. 
Physical Barriers. 
1. Density . 
It is a well-known fact that many plankton organisms — both 
animals and plants — show a seasonal variation of form which is 
in the direction of a pronounced increase of surface in the 
summer months. Species of the genus Daphnia, for instance, 
have in summer smaller bodies, long head crests and long 
spines, whereas in winter the body is larger, the head rounded, 
and the spine short. It has been very clearly shown that this 
change accompanies a change in what is called the viscosity of 
the water. The viscosity of the water decreases greatly with a 
small increase of temperature and to a less extent with its 
dilution, so that a floating organism re-acting to such a change 
must either greatly increase its muscular action or its surface of 
resistance to the water in order to remain floating. 
It is evident, therefore, that a marine form entering fresh 
water with its less density and its greater changes of tempera- 
ture must find great difficulty in remaining afloat. It appears 
to me that fresh water must, in this sense, present an almost 
insuperable obstacle to the immigration of plankton. 
But density and varying viscosity are not insuperable barriers 
to animals which are adapted to life on the bottom or among 
weeds, and the great majority of the more recent marine 
immigrants are either more or less sedentary forms or are 
powerful swimmers. 
