132 Origin of the Araneidal Fauna of Yorkshire. 
been arranged in five faunistic groups, according to the direction 
from which, as the circumstances of their present range 
indicate, they have severally diffused, but it should be borne in 
mind that, from the difference of geographical position and the 
intervention of organic and physical barriers, the conclusions 
proper to one area will not necessarily apply in their entirety to 
another. Also there are species which from our defective 
knowledge of them, or the peculiarity of their present distri- 
bution cannot be safely assigned to any one group. 
All the sections are more or less numerically represented 
in the Yorkshire fauna. 
NORTHERN GRoupP.—A considerable portion of the County’s 
spiders, most of which are abundant, at least in its hilly western 
portion, belong to this section, e.g., Coelotes atropos Walck, 
Bolyphantes alticeps Sund., B. luteolus Bl., Leptyphantes 
eviceus Bl., Poeciloneta globosa Wid., Hilaira excisa and uncata 
Camb., Macrargus rufus Wid., Phaulothrix hardu Bl., Mengea 
scopigera Grube, Centromerus arcanus Camb. C. prudens Camb., 
Microneta gulosa L. Koch., Evigone arctica White, Caledonia 
evanstt Camb., and others. They are much commoner in Scot- 
land and in the North of England, but are altogether absent or 
very rare in the south of this island. Many reappear again on 
the Continent and penetrate in some instances a considerable 
distance to the south. The reason why they have not success- 
fully colonised the south of England is not understood, but it 
is possible that the more highly cultivated state of the land and 
the drier conditions which there prevail (the ground moisture 
depending not so much on the rainfall as on the permeability 
or otherwise of the geological formations) and the consequent 
restricted area of wild tracts which yield the moist habitats in 
which they thrive best, together with their inability to maintain 
their ground against other species more tolerant of crowding and 
competition, but less hardy of constitution, may have had 
something to do with it. This explanation is in some measure 
supported by the fact that many of them when occurring in 
the south of England and on the continent are found on the 
higher uncultivated levels. .As tothe genesis of the northern 
species there is room for difference of opinion, but whether 
they are believed to have participated along with other organ- 
isms. in the migrations southward and then northward, con- 
sequent on the gradual approach and departure of the Ice 
Age, or as there is more reason to believe seeing that such a 
theory explains more satisfactorily not only the present 
disposition of the faunistic groups, but also the sameness of the 
genera and frequently even of species in the colder regions of 
both hemispheres, that they radiated from a common centre 
somewhere in the north at-a period subsequent to the Glacial 
Epoch, they are probably the oldest members of the araneida] 
Naturalist, 
