362 THE FRESH-WATER LOCHS OF SCOTLAND 
we are fairly safe in saying that such sensitive forms must be restricted 
in their range, and are Httle hkely to colonise fresh water. 
The evidence which has been cited seems to su^iffest that our lists 
of fresh-water and salt-water forms, while indicating correctly the 
general tendencies of the groups in question, are liable to be modified, 
as exploration brings to light types which are adapted to a different 
state of existence. We are justified in saving that in most instances 
it is not really impossible for representatives of this or that group to 
exist in either fresh or salt water as the case may be, for increase of 
knowledge has repeatedly brought to light cases which are exceptions 
to the ideas previously held. Our proposition that fresh-water forms 
have been derived from the ocean is clearly supported by the evidence 
we have at our disposal, and a good deal of this concerns a transfer- 
ence from sea-water, as we now know it, to water which is brackish 
or fresh. 
We have, however, no reason to suppose that the water of the 
ocean has always been just as saline as it is at present; indeed, we 
have everv reason to believe that its salinity has been slowly increas- 
ing through countless ages, by the addition of salts dissolved out of 
the land-masses. Quinton has collected testimony to prove that, on 
the one hand, the sea of former epochs was essentially the same in 
chemical composition as that of to-day,^ but that, on the other hand, 
,the concentration of the salts in the water was very considerably less.^ 
If, then, we know of organisms which have been able to accomplish a 
greater change in recent times, it is not hard to believe that many 
forms gradually achieved a lesser change during past geological ages. 
A further discussion of this is not necessary here ; but granting 
that the earliest known forms of life were inhabitants of the ocean, 
and that the non-salinity of rivers and lakes was, in most cases, no 
insuperable bar to colonisation, we have to look for other reasons 
which may explain why only certain forms (and a very small 
assemblage, in the case of animals) have succeeded in establishing 
themselves. There are, indeed, other factors which have had as great 
or even (greater influence in hinderincr the micrration into fresh water 
O OCT 
as the difference in salinity, and these we may proceed to enumerate. 
In the front rank we may place the prevalence in the sea of 
delicate, feebly-swimming organisms, or forms having weak free- 
swimming larvae, for it is obvious that these could not contend against 
the seaward current of rivers and streams. The very exceptional 
occurrence of jelly-fish in fresh water is, for instance, probably due to 
1 Op. cit., p. 235. 
2 Ibid., p. 446, The figures given are 3 "5 j^er cent, of dissolved salts, as an 
average for the existing ocean, and 0'85 per cent, for the primitive ocean in which 
we believe life to have originated. 
