466 
president’s address. 
in the Devonian period than it has been since, for the contrast 
between the conditions of life in the sea, and in fresh water, 
cannot then have been the same as it is now. There can be no 
doubt that the sea was not so salt then, and it is attractive to 
suppose that an extensive immigration from the sea took place 
when the sea was only slightly more salt than the water 
flowing into it. However, it must be remembered that the 
first water on the earth must have been very hot, and have 
possessed great solvent power, and also that immense periods 
of time elapsed before the development of highly organised 
animal life, during which time the sea was becoming rapidly 
salter. It is calculated by Arldt 1 that the salinity of the 
Cambrian Sea had already reached between 2‘7 and 2‘79 per 
cent., so that during the life period of, at least, all the higher 
forms of life, the contrast between sea and fresh water can 
never have been considerably less than it is to-day. It seems, 
therefore, that the colonisation of fresh water must have been 
effected in the past under approximately similar conditions to 
those which obtain at present. 
THE NATURE OF THE BARRIERS BETWEEN 
FRESH AND SALT WATER. 
In the existing state of the world we find profound physical 
and chemical differences which might appear to be an insuper- 
able barrier between sea and fresh water, and yet we know that 
this barrier has been passed, and we believe that even now it is 
being passed by some animals. It becomes, therefore, a problem 
of some interest to consider what are the powers of resistance 
of living forms to these differences, and how, in fact, they have 
been surmounted. 
The differences in the conditions to which animals are 
exposed in fresh and salt water fall under the heads of 
Chemical and Physical, and it is the chemical difference w'hich 
is the most important, and first claims our attention. 
1. Entwicklung der Kontinente, 1907, p. 550. 
