NATURE AND ORIGIN OF FRESH-WATER ORGANISMS 369 
the further development takes place, aided probably by nutriment 
obtained from the host by means of outgrowths penetrating the 
tissues. When the young mussel is fully formed, the cyst bursts, and 
the mussel falls to the bottom to assume a sedentary life. Whether 
this wonderful method afforded the means by which the mussels were 
enabled to colonise fresh water is doubtful. It is more probable 
that the process has been entirely evolved since they assumed a fresh- 
water habitat, and that its object has been to assure adequate distri- 
bution within the limits of that medium. 
We have now to deal with the second method by which organisms 
primarily marine have come to inhabit fresh water, namelv, by 
becoming terrestrial or swamp-loving in nature, and secondarily 
adapting themselves to a fresh-water life. In the first place, we 
are fully justified in supposing that the forms belonging to groups 
overwhelmingly terrestrial in character come into this category, and 
are modified land forms and not direct immigrants. This is in all 
probability the case with the majority of the higher plants ; indeed, 
amongst the Angiosperms there are species living in swampy sur- 
roundings which can perfectly withstand changes from an almost 
wholly terrestrial to a partially submerged existence. 
It is equally obvious that the Mammalia are an essentially 
terrestrial group, and that therefore fresh- water mammals (as, of 
course, marine mammals) have become secondarily adapted to a very 
different mode of life. The same is presumably true of the insects 
and arachnids which are now constituents of the fresh-water fauna. 
Finally, we may mention certain fresh-water Gasteropods, which 
belong to the great group of air-breathing forms (the Pulmonata) and 
so may be supposed to have secondarily arrived in our rivers, lakes, 
and swamps. Among the most common genera belonging to this 
group are Limnoea^ Planorbis^ and Ancylus. 
The third method by which fresh-water organisms have been pro- 
duced — by the isolation and freshening of portions of the sea — is a 
wholesale method, which must have acted upon a number of most 
diverse forms. It is, of course, clear that in a basin isolated by earth- 
movements from the sea there would probably be many organisms 
totally unable to accustom themselves to a fundamental change in 
salinity, however gradually that change might be accomplished. 
There are certain indications afforded us as to which forms could 
survive, both by the experiments of Beudant and by the instances of 
partial direct colonisation by marine types which have already been 
discussed. We have also seen sufficient evidence that this process has 
actually been at work, but it is nevertheless practically impossible to 
point out the groups which have become inhabitants of fresh water in 
this manner. The temptation is to assume that all the organisms in 
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