NATUflE AND ORIGIN OF FRESH-WATER ORGANISMS 373 
which, while not prechiding differences of a minor degree, is far in excess 
of that observable in the ocean, where the conditions of life are so 
profoundly distinct ? 
While the incessant dispersal of forms tends to hinder the creation 
of new varieties and species, it must tend to produce types which are 
hardy and adaptable, and therefore thoroughly fitted to survive. In 
this connection a suggestion has been put forward which goes a little 
further than we have ventured, but without denying the essential 
truth of our assertion. It is that the uniformity of fresh-water 
organisms is due to the persistence of the hardy adaptable types of 
cosmopolitan distribution, and the dying-off of local types, as a 
result of altered conditions. The local varieties, which under more 
favourable circumstances might have attained specific rank, are 
regarded as having frequently succumbed to the combined effects of 
changing conditions and the relentless competition of more adaptable 
generalised forms. It may be so ; but, although our present knowledge 
of this intricate problem is very far from complete, there is no need 
to look beyond the exceptional capacity for dispersal for an explana- 
tion of this phenomenon. 
This, then, is the conclusion we arrive at respecting the general 
uniformity of the organisms of fresh water. We are able also to 
assert confidently that fresh-water organisms are the modified descend- 
ants of marine ancestors, and we have indicated at considerable 
length the different ways in which we conceive the colonisation of 
inland waters to have taken place. On the earlier question we set 
ourselves — that of the existence of only certain forms in fresh water 
— it is more difficult to reach a conclusion. Many causes have been 
enumerated, each of which may have had its effect in preventing this 
or that type from leaving the littoral zone for the ijiland waters of a 
continent, but we are still confronted with exceptions which we 
cannot explain, both of forms which have unexpectedly succeeded in 
migrating, and of others which have incomprehensibly failed to do 
so. We are, in fact, face to face with some of the most profound of 
Nature's problems, and while we may safely predict that increasing 
knowledge will throw light upon many obscure matters, the time is 
far hence when we shall be able to unravel the complex effects pro- 
duced on living matter by the influence of animate and inanimate 
surroundings. 
