370 
THE FP.ESH-WATER LOCHS OF SCOTLAND 
our fresh-water flora and fauna not easily accounted for by one of the 
other methods are the modified descendants of those left in detached 
marine basins, but there is no justification for such an extreme view. 
Speaking broadly, we may say that those forms which are too feeble 
to migrate actively from the sea, and are unprovided with any means 
for securing their transport passively, are those which we should 
account for by this third method. It is very evident, however, that 
these questions involving the relations of organisms to their surround- 
ings are of extreme complexity, and demand a degree of knowledge 
far beyond the small beginnings hitherto made. In many instances 
we may be suggesting theories which are quite far-fetched, and sub- 
sequent discovery may show much simpler explanations. 
We turned our attention in the first instance to the very striking 
differences which exist between marine and fresh-w^ater organisms, no 
matter in what quarter of the globe we compare them. If we turn 
now to a comparison between the fresh- water organisms of different 
parts of the world, we find an equally striking similarity between 
them. This uniformity of fresh- water organisms, sufficiently marked 
when we knew little outside the bounds of Europe, has become more 
and more strongly emphasised as information has been collected from 
the remoter regions of the world. It is not asserted that forms from 
widely separated fresh-water areas are necessarily identical, but we 
frequently find generic, and sometimes specific, resemblances, while 
there is a general uniformity far more pronounced than any to be 
observed in marine organisms. There are, of course, differences of a 
minor nature due to differences of climate, and these we must treat 
of in detail elsewhere ; but we are concerned for the moment only 
with the very natural query : Why does this very definite uniformity 
exist ? 
Some of the facts which appear to offer a clue have already been 
indicated. We have examined at some length the dangers and 
difficulties to which forms colonising fresh water are exposed, and have 
pointed out the means adopted by different groups for overcoming 
them. Knowing this, we can explain why certain types only are to be 
found in all the fresh waters of the globe — they alone have been able 
to adapt themselves to the peculiar conditions. But when we leave 
on one side the actual origin of fresh- water life, and study the 
agencies which have secured the distribution of these forms from one 
centre to another, we gain more light on our problem at once. 
It will be remembered that, under the heading of active and 
passive migration from the ocean, several methods were referred to 
which would be equally capable of effecting a general distribution 
within the limits of fresh water. It was no part of our proposition 
