DISTRIBUTION BY ENVIRONMENTS 

105

that about sixty species are restricted in Britain so far as is known to 

areas south of the 50  F. line, and about 190 to south of the 48  F. 

line, whilst a fair number do not extend southwards as far as the 48  

line. 

Temperature and humidity are the most important factors, but the 

potential range of distribution of a species is decided by a host of 

complex considerations which it is difficult for us to assess at their 

true value. Maximum temperature, minimum temperature, the 

temperature at particular seasons, the length of the seasons, the 

rapidity and extent of temperature changes, humidity, rainfall, light 

intensity, wind, the physical properties of the soil, the nature of the 

vegetation, etc., all have their effect. For every species there will be a 

minimum and a maximum limit for each factor, but in practice some 

extremes of tolerance may be extended beyond the theoretical limit if 

the other factors are optimum. 

A point of great importance to bear in mind is that many climates 

may prevail at the same time within quite a small area. Mountain tops, 

caves and the interior of houses are obvious examples, but it would 

hardly be an exaggeration to say that every kind of plant will provide 

its own particular local climate for the creatures which live on it or at 

its roots. Mellanby (1933) has shown that the temperature at grass 

roots, in rat holes, in moist soil, etc., may be several degrees lower 

than that of the air temperature just above. 

Some of the different environments frequented by spiders will be 

discussed in the pages which follow. 

PLANT PREFERENCES. 

In a general way the collector of spiders comes to realize that some 

plants harbour more spiders than others. He may, for instance, be 

disappointed in his sifting, sweeping or beating of Bracken, Bluebells, 

Mint, Rosemary, Lemon Verbena, Laurel and the foliage of Alder, 

Willow and Beech. His haul will be richer from