INTRODUCTION 3 



More important from many points of view is the fact 

 that the seas are of vast size and, for the most part, 

 continuous ; while the fresh-water areas are relatively 

 inconsiderable in extent, and broken up into count- 

 less small areas. Every lake and river-system is an 

 isolated territory, and most of the plants or animals 

 found therein can only make their way into another 

 territory by help of a rare opportunity. This par- 

 ticular circumstance has governed the course of life 

 to a material degree, and has brought about some 

 of the most marked differences between the fluviatile 

 and the marine populations. It is also worth while to 

 notice that in the large fresh-water areas, which in 

 nearly every part of the world are the rivers, the 

 water is in motion, and flowing steadily towards the 

 sea. Immersion in the sea is quickly fatal to most 

 fresh-water animals and plants, so that here is a 

 special danger to which the smaller and more help- 

 less fluviatile animals are exposed. We have reason 

 to believe that this special danger has helped to bring 

 about one of the most noteworthy peculiarities in the 

 life-history of fluviatile animals, viz., the suppression 

 of free-swimming and ciliated larval stages.^ 



The Invasion of the Waters by Insects. 



The waters, both fresh and salt, have been success- 

 fully invaded by Insects, and this not in one or two 

 cases only, but many times over, and by the most 

 diverse kinds of Insects. The aquatic Insects do not 



1 This has been pointed out by Professor Sollas (Proc. Roy. 

 Dublin Society, 1884). 



B 2 



