112 Origin of the Araneidal Fauna of Yorkshire. 
‘of probability from what quarter they entered and in what 
direction they spread over the British Isles. As their geological 
records show, spiders have an equal antiquity and must 
necessarily have come under the operation of the same natural 
laws and been acted upon by the same influences, and we may 
therefore feel sure that they came to this country in a similar 
way to their larger and better known neighbours. 
The most generally accepted theory of plant and animal 
distribution, and the one which most accurately accords with 
known facts is that of Prof. E. Forbes’s* specific centres from 
which the different forms diffused as organic and physical 
conditions permitted to the surrounding districts; utilising 
as a means of access to the British Isles those land connections 
which anciently united them at different times to adjacent 
parts of the Continent, but which subsequently were entirely, 
or as in the case of the one to the north, of which the highest 
points, now known as the Orkneys, Shetlands, Faroe Islands and 
Iceland still remain above the sea, only partially submerged. 
It is probable that, since the creation of the existing fauna and 
flora, several such oscillations of levels—long periods of de- 
pression and elevation—have occurred, each producing impor- 
tant modifications in the distribution of land and sea and 
undoubtedly also giving rise to corresponding variations in 
temperature and in the quantity and duration both of winds and 
rains. Any one, or combination of new conditions, whenever 
they obtained cannot but have exercised the greatest and most 
far reaching effects on the various organisms, favouring some 
which would rapidly increase in numbers and overrun large 
tracts of land, but adversely affecting others which would as 
quickly decrease and eventually give way before the more 
vigorous and dominant forms. 
Physical causes alone, however, are not sufficient in them- 
selves to account for the present distribution of plants and 
animals. Other influences not of a material kind like those 
already enumerated, have had an equal if not greater share in 
determining the limits of their extension. Just as the organism 
which failed to adapt itself to the demands of its physical 
environment, would give way before one which could, so would 
it also be eliminated if it failed to respond to the co-existng 
organic conditions while the more adaptive species would in- 
crease and flourish. These directing forces are still operative, 
imperceptible as of yore, and chief amongst them in its power 
to influence distribution is that keen competition for a due 
share of the food so vital to every form of life, which is the 
result of the strenuous conditions of overcrowding, which, 
induced by the enormous multiplication of individuals, obtains 
* Flora and Fauna of the British Isles. Mem. Geol. Survey. 
Naturalist, 
