366 
THE FRESH-WATER LOCHS OF SCOTLAND 
have been already indicated. The horny-coated resistant eggs, to 
which reference has been made, are structures characteristic of various 
fresh- water animals. The suppression of a free-swimming larval 
stage in certain cases may be accompanied by structural changes in 
the parent, having as their object the protection of the ova. Again, 
there is less need for stout protective armour in fresh water (particu- 
larly in ponds), so that fresh-water Gasteropods, for example, are 
usually distinguishable by their thin shells from their marine allies, 
which are fitted to withstand the breakers of the sea-shore. 
In pointing out, however, the external characteristics which are 
directly due to the conditions under which these animals live, we 
would strongly emphasise the necessity for excluding such features as 
far as possible, when deciding the systematic position of any animal. 
It is only by doing so that we can gain a satisfactory idea of the 
true interrelationships of forms some of which have remained permanent 
inhabitants of the ocean, while others have secondarily become adapted 
to life in fresh water. 
Having examined in some detail a number of facts which bear 
directly on the colonisation of fresh water from the sea, we must now 
proceed to consider the means by which this process actually took 
place. It is obvious that fresh-water organisms must have attained 
their present distribution in one of three ways : (1 ) by a direct, 
active or passive, migration from the sea ; (2) by becoming terrestrial 
or swamp-loving in nature, and secondarily adapting themselves to 
life in fresh water ; (3) as a result of the isolation and subsequent 
freshening of some portion of the sea, due to movements of the earth's 
crust.^ No doubt fresh-water organisms have been derived from marine 
by all three of these methods, but it is by no means easy to assert which 
of them has played the most important part. In passing to the con- 
sideration of the methods in more detail, we must seek to determine 
whether the known fresh-water forms possess characteristics which 
would fit in with the suggested explanations, and we may also indicate 
the particular manner in which the more important groups achieved 
this material change in their environment. 
Treating in the first instance the subject of active migration,^ it is 
clear that this means is only open to strongly-swimming forms or to 
such as walk or crawl on the bottom, for these alone would be able to 
invade rivers from their mouths, and so effect a permanent settlement 
within them or within any associated basin of water. We think at 
1 Cf. Sollas, "The Origin of Fresli-water Fauna," in The Age of the Earth, and 
other Geological Stn.dies, London, 1905, p. 178. 
2 We are dealing for tlie moment only with the emerging of marine types to 
become members of a purely fresh- water series. Migration from one area of fresh 
water to another is a separate question. 
