DISTRIBUTION OF ANIMAL COMMUNITIES 13 



woodland and in wood margin, shrub communities, and young 

 plantations. The actual figures for comparison with Apodemus 

 are as follows : 47 per cent, of the specimens in woods and 

 53 per cent, outside, chiefly in shrub or young tree habitats. 



It may be, in fact, often rather an arbitrary proceeding to 

 split up the animals living on, say, a mountain-side into com- 

 munities corresponding to the exclusive and characteristic 

 species of each life zone, and it should be realised quite clearly 

 and constantly borne in mind when doing field work, that 

 many common and important species come in more than one 

 zone. Richards, after several years' study of the animals of 

 an English heath, says : " The commonest animals in a plant 

 community are often those most common elsewhere." ^^^* At 

 the same time it is probably true that animals living in several 

 zones of vegetation show a marked tendency to have their 

 limits of distribution coinciding with the edges of the plant 

 zones. This is only natural in view of the step- like nature of the 

 gradient in environment produced by the plant communities. 



II. Another important vertical gradient is that found in 

 the sea and in fresh- water lakes, and this is caused by the 

 reduction in the amount of light penetrating the water as 

 the depth increases. This gradient shows itself both in the 

 free-living communities (plankton) and in those living on the 

 bottom (be?ithos). As we go deeper down the plants become 

 scarcer owing to lack of light, until at great depths there are 

 no plants at all, and the animals living in such places have to 

 depend for their living upon the dead bodies of organisms 

 falling from the well-lighted zone above, or upon each other. 

 There is the same tendency for the plants to form zones as 

 on land, and one of the most interesting things about marine 

 communities is the fact that certain animals which have 

 become adapted to a sedentary existence compete with the 

 plants (seaweeds of various kinds)- and in some cases com- 

 pletely dominate them. The reason for this is that in the sea, 

 and to a lesser extent in fresh water, it is possible for an animal 

 to sit still and have its food brought to it in the water, while 

 on land it has to go and get it. Web-spinning spiders are 

 almost the only group of land animals which has perfected a 



