630 
Louping III. 
remain unburied or unburned, during the dissolution of the carcass 
numbers of the specific microbes have an opportunity for becoming 
scattered and distributed over the soil, to remain there ready for 
new mischief when the favourable time (season) and opportunity 
(the susceptible animal) offer themselves. Conditions like these 
would readily account for and explain how the soil of a locality 
once infected remains a source of infection with great persistence, 
and also how a locality free from diseased sheep remains free from 
the infection, while an adjoining farm year after year harbours the 
disease. 
There exists among the farmers and shepherds in the affected 
districts of Northumberland and elsewhere, a strong, and I may say 
an almost universal, opinion, shared in by many others who live in 
the district, that the disease is caused by a true tick — a poisonous 
species of the Arachnida known as the “ white tick” as opposed to 
the ordinary or black sheep tick, and this opinion is founded on 
the following assertions : (a) this “ tick ” is present where the disease 
occurs, but (b) is absent where there is no disease or where the 
disease is unknown. About the first, viz., that where there is disease 
there is also the tick, there can be no doubt : the inspection of the 
skin (axilla, groin, and thigh) of a diseased sheep or lamb proves 
this ; but I must confess that I have grave doubts as to the correct- 
ness of the second, viz., the absence in all cases of the tick from 
localities where there is no “ louping ill,” and my doubts will, I am 
afraid, not grow less strong the greater the confidence with which 
that assertion is put forth. Unless, and not until, this statement 
is supported and confirmed by authority, I mean by a zoologist well 
qualified to give an opinion — until it is really proved that this tick 
does not exist in localities where the disease does not occur, e.g. on 
similar moorlands of Yorkshire and Cumberland, I must hold 
that the conclusion arrived at by the farmers and shepherds and 
other laymen is based on the well-known fallacy in argument : post 
hoc ergo propter hoc ; in other words : “ The tick is found in a 
locality where disease prevails, therefore the tick is the cause of the 
disease, and therefore where there is no disease there is no tick.” 
My doubts in this matter are, in the first place, prompted by 
the remarkable and undeniable fact that the same tick is present in 
an infected locality on a good many sheep that are, and remain, free 
from the “ louping ill ” ; this is admitted by the same persons who 
consider the tick as the primary cause of the disease ; and, in the 
second place, it would be almost short of a marvel that the tick 
should be present in one farm and, by the farm-boundary — a loose 
stone fence or less — should be excluded from the next farm, which 
is free from the disease. I say such an explanation of things seems 
to me almost impossible of acceptance. There is certainly nothing 
known in the whole range of epidemic diseases that would offer a 
similar remarkable condition. An animal that is affected with the 
disease, as a rule, shows in the axilla or groin or thigh one or two 
ticks well buried with the head and legs within the skin ; in some 
cases the tick is no longer there, but has left a mark — a blood 
