8 THE BIOLOGY OF THE SEA-SHORE 



few birds venture far from land, and insects, so important 

 a part of the aerial fauna above a land surface, with few 

 exceptions, do not venture beyond tide-marks. Among 

 the exceptions must be noted the group of insects known as 

 the Halobatidae. Further, swarms of migrating insects, 

 such as locusts and butterflies, may be blown out to sea by 

 gales and, with the help of the wind, may cross many miles 

 of intervening sea. It is in this way that many insect forms 

 have been distributed, and that foreign stragglers continue 

 to reach our shores. The occurrence of these forms, 

 however, above the open sea is obviously quite fortuitous. 

 The flying-fishes, by taking a strong leap and using their 

 pectoral fins as parachutes, are able to sustain themselves 

 for short periods above the surface of the ocean. His- 

 torically, the aerial habitat was the last to be invaded 

 by animals. 



The Aquatic Habitat. — This we can at once divide into 

 a fresh- water and a marine habitat, the latter containing 

 an enormously greater number of forms. Fresh- water 

 faunas are distinguished by their uniformity. Thus, while 

 the population of a fresh- water lake may be very dense, 

 the number of species is relatively small and the number 

 of types still smaller. In considering the conditions of life 

 in fresh water we are able to distinguish forms living in 

 pools and in running water, and another subdivision might 

 be made into littoral, surface, and deep-water forms. Lull 

 {loc. cit), however, remarks that although in a measure the 

 deep-sea characteristics of darkness and great pressure 

 occur in certain lakes and relict seas, yet no profound modi- 

 fications of form occur comparable with those seen in the 

 animals of the marine abysses. The reason for this, Lull 

 thinks, is to be found in the comparatively evanescent 

 character of lakes from the geological point of view. 



Of all abodes of life the marine habitat is biologically the 

 most important. The fauna is not only remarkable for its 

 numbers, but still more for its extraordinary diversity. 

 We may recognise the four following divisions : (i) Strand 

 or Tidal Zone, (2) Shallow sea, (3) Pelagic, (4) Abyssal, 



