Origin of the Avaneidal Fauna of Yorkshire. 137 « 
elimination of species and ‘individuals. Works of the first- 
named description have in the Wessenden Valley removed the 
habitat of Tugellinus furcillatus Menge, and in the Chew 
Valley (S. W. Yorks), that of Huillhousia misera Camb., 
with the result that neither of these rare species have been seen 
since in these localities though fortunately both subsequently 
turned up in other parts of the county, so that we have 
not to deplore their total loss. 
Hens, which are now so extensively kept, very often with 
a free run in those places most likely to be frequented by spiders, 
make a wonderful clearance of all the minute forms of life. 
In the little cloughs amongst the hills given over to poultry, 
spiders can only be met with in places inaccessible to the hens, 
such as tangled masses of ground-growing thorns and brambles, 
vertical banks, beneath stones, etc., but in neighbouring 
cloughs of precisely the same description where none is kept 
there is no such restriction, the spiders being more generally 
distributed. 
So great indeed has been the effect of man’s long continued 
and unceasing labour that in course of time he has, in one way 
or another, more or less completely altered the surface aspect 
of this country, and in doing so, must, as I have endeavoured 
to show, have affected the composition of its fauna (inclusive of 
spiders) to an incalculable extent. Depending on this circum- 
stance alone, there is nothing inherently untenable in the 
proposition that, in the original undisturbed condition of the 
country, the southern species may have been able to make 
greater headway than is indicated by their present range, 
that some have more or less successfully adapted themselves to 
the changing environment and are maintaining their ground, 
while others have failed to do so and been driven back towards 
their old territory, so that such species as Diga dorsata Fabr., 
(now lost to Yorkshire), Micrommata virescens Clerck, Cercidia 
prominens Westr. and Crustulina guttata Wid., which have 
occurred in places, very little if any changed from their primeval 
state (situations which negative the idea of their accidental 
introduction), may be lingering survivals of such an extension 
rather than species which have succeeded in penetrating 
farther north than their compeers. 
The other unexpected forms, Clubiona subtilis L. Koch., 
Protadia subnigra Camb., Linyphia impigra Camb., Hyctia 
mwoyt Luc., Euophrys equipes Camb. in the East Riding ; 
Syedra pholcommoides Camb., Coryphaus simplex F.O. P. Cb., 
Trochosa robusta Sim. in the North Riding ; Coelotes terrestris 
Wid., in the North and West Ridings ; Clubiona facilis Camb, 
in the West Riding, give no more hint of the method of 
their advent into the county than can be gathered from 
the fact that, with one exception, they were found on the 
K 
1913 Mar. 1. 
