4 THE BIOLOGY OF THE SEA-SHORE 



turning back on meeting with unfavourable conditions, 

 (3) by selection after trial in connection with migration. 

 It is because of physiological similarity that numbers of 

 animals select the same environment. Thus agreement 

 in behaviour and physiology among the members of an 

 animal community is to be expected, just as there is 

 growth and form agreement in plant communities. 



What are the chief factors controlling the distribution 

 of plant and animal life .'' In the first place we have 

 the physical factors as exemplified by the three primary 

 habitats : air, land, and water. If animals are considered 

 in their relation to these three great environments, it 

 will be seen that although air is essential to life, yet 

 animals are never purely dwellers therein. A great many 

 animals are essentially restricted to a land life, requiring 

 air to breathe and a more or less solid substratum for 

 their activities, while others are equally restricted to 

 the fluid medium, either with or without a substratum. 

 There are some that can utiHse all three media to some 

 extent, as in the case of some birds which can fly, walk, and 

 dive. To go somewhat further, we find many animal forms 

 restricted to particular well-defined types of physical con- 

 ditions and unable to live normally, or at all, beyond their 

 influence. We can thus distinguish animals peculiar to 

 warm, temperate, and cold waters or climates, to wet or dry 

 regions, to light or dark situations, to deep or shallow waters, 

 to lowlands or mountains, to swift or stagnant waters, and 

 so forth. With regard to aquatic communities, an interest- 

 ing experimental study of the behaviour agreement among 

 the members of communities has been made by Shelford 

 {op. cit.). For the purpose of contrast the societies of rapids 

 and of pools of a stream were chosen, and their reactions 

 to various stimuli tested in the laboratory. The behaviour 

 in relation to light, currents, etc., was found to correspond 

 strikingly with the conditions under which the animals 

 lived. Another fundamental factor governing the occur- 

 rence of animals is that of food. Many animals are wholly 

 vegetarian, and hence their distribution is bound up with 



