44 ANIMAL ECOLOGY 



exact type of habitat frequented by the animal, and perhaps 

 carry the work still further. There is a species of spider 

 (Leptyphantes sohrius) living in Spitsbergen, which illus- 

 trates this method. Leptyphantes sobrius inhabits a great 

 many types of country, occurring high up on the sides of 

 mountains, or on lowlands, or even on the sea- shore, both in 

 damp and dry places, and it does not appear to be very con- 

 stantly associated with any particular species of animal or 

 plant. At first sight we might describe its distribution as 

 universal, but careful notes on the habitat of every specimen 

 collected show that there is one feature in its distribution which 

 is absolutely constant. It invariably comes in places where 

 there are rather unstable patches of stones, in stream gullies 

 or on screes, or on the shores of ponds and lakes or of the sea. 

 If there is a patch of soil in one of its typical habitats which 

 has become stabilised through the agency of plants and is no 

 longer liable to slide away or be washed away during the spring 

 snow-melting, then Leptyphantes disappears, or at any rate 

 its numbers will be relatively very small, and other species of 

 spiders (such as Typhochrestus spetsbergensis) will take its place. 

 It is worth while in a case like this to be able to define the 

 habitat of the species by some definite factor which is always 

 associated with the animal's distribution, even though, as 

 with Leptyphantes, we do not know in the least what the im- 

 mediate reason for this distribution can be. But it should 

 always be borne in mind that such descriptions of habitat 

 are only a temporary expedient to assist in discovering the 

 real habitat, wliich must be defined in terms of limiting 

 factors. These rough habitat- descriptions are also of great 

 use, since they enable one to make records which assist in the 

 working out of the animal communities of different general 

 habitats. 



13. Although we have said that a knowledge of physiology 

 is not always necessary for the solution of ecological problems, 

 it is sometimes of use, especially in cases where the animals 

 with which we are dealing belong to the class which finds its 

 habitat by broadcasting large numbers of young ones, so that 

 only those survive which find a suitable spot. This is true of 



