NATURE AND OPJGIN OF FRESH-WATER ORGANISMS 365 
bodies, called by various names, are to be met with in several different 
groups, as we have already seen, and in certain cases where two dis- 
tinct methods of reproduction exist, it is incipient drought alone which 
causes the production of these bodies before the adults succumb to the 
impossible conditions. In other cases, notably among the Cladocera, 
there are two fairly well-marked periods during which specially re- 
sistent ova are produced, the one during the summer, as a precaution 
against desiccation, and the other at the beginning of winter, to ensure 
protection from the frost. 
In a manner perfectly analogous to what we have seen in the case 
of cold, it is found that the eggs of a species of Apus will not develop 
unless they have been dry for a considerable period.^ 
The most important reasons why fresh water has not proved easy 
to colonise have now been discussed, but there remain a few other 
points to be indicated, which may doubtless exert an influence at 
times. Organisms are directly affected by their interconnection with 
each other. That is to say, in certain cases they are dependent on 
one another to such a degree that the absence of one entirelv pre- 
cludes the presence of another which might otherwise be perfectly 
able to adapt itself to new conditions. This may be a matter of food- 
supply : a higher animal, for example, cannot extend its range into a 
medium in which its food, whether animal or vegetable, does not 
exist, so that if from any cause a river or lake were conspicuously 
deficient in this respect, it would stand little chance of receiving 
voluntary migrant forms from the ocean. There may be also a less 
obvious interdependence, concerning protection and shelter for a 
defenceless type. 
Lastly, any impurity of the water of streams and lakes would act 
as an efficient barrier in many cases. The impurity might be merely 
mechanical, and due to large quantities of mud held in suspension, or 
chemical, and caused by the presence of salts or acids in solution. 
Examples of the former are well known, where during certain periods 
of the year rivers become well-nigh uninhabitable. Other rivers, 
and more particularly lakes, may carry in solution unusual quantities 
of lime or manganese salts, or may contain a considerable admixture 
of humic acid, and these conditions would be unfavourable to the 
majority of ocean types. 
Before passing to other considerations, it may be well to again 
call attention to the fact that certain fresh-water organisms exhibit 
structural peculiarities which have undoubtedly been produced by 
existence under non-oceanic conditions. That is to say, actual 
morphological features have been created which in many instances 
enable them to be recognised as fresh-water forms. Some of these 
^ Semper, op. cit., p. 175. 
