DISTRIBUTION BY ENVIRONMENTS
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
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.
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