86 



LIFE : OUTLINES OF GENERAL BIOLOGY 



alternation of day and night, felt much more markedly than on the 

 open sea, the endless variations between gently lapping waves and 

 blasting breakers, the slow changes of subsidence or elevation — 

 these are some of the vicissitudes to which shore animals are exposed. 

 The shore is rich in illustrations of keen struggle for existence and 

 of life-saving shifts or adaptations, such as masking, protective 

 coloration, surrender of parts, and "death feigning". We may think 

 of it as a great school where many of the primary lessons of life, 

 such as moving head foremost, were learnt. 



The Vkky Varikd Fres!Iwatkr Faina. — Compared with the 

 total surface of the earth (197,000,000 square miles) the fresh 

 waters occupy but a small area (1,800,000 square miles), i.e. a little 

 over one per cent., yet they exhibit great variety. One has only to 

 think of lakes and tarns, ponds and pools, wells, ditches, streamlets. 



Fig. 22. 

 .\ Typical Littoral Fish [Cottus Scorpio), or HuUiicad. IVom a specimen. 



and rivers, to call up pictures from this world-wide heterogeneity: 

 and in the lake at least, geologically distingui.shed from the pond 

 not by size but by depth, it is often necessary to distinguish littoral, 

 open-water, and abyssal zones. Thus parts of Lake Baikal have a 

 depth of 760 fathoms, compared with which the vast Caspian is 

 almost a pond. 



In the eighteenth century there was an enthusiastic and illumin- 

 ating study of freshwater animals, led by men like Lceuwenhoek, 

 Koesel von Koscnhof, Reaumur, and Trembley, who.se industry and 

 insight are well appreciated in the late Prof. Mialls Early Naturalists. 

 With the Ix'ginning of the nineteenth century, however, the attrac- 

 tiveness of marine zoology Wgan to make itself felt, and soon claimed 

 the attention of the majority. Yet after a long period of relative 

 neglect the freshwater fauna is once more the subject of lively 

 interest, the revival having been in no small degree due to the 

 thoroughness of Mialls studies on the life-histories of freshwater 



