39 
perhaps become then very sensibly felt, and their powers be 
changed. That they contribute to health in some degree is, I 
think, very probable, for notwithstanding the apparently un¬ 
necessary existence and cruel effects of the Oestri, they are pro¬ 
bably not altogether without an use, and were not designed by 
Providence to add, without a recompense, to the numerous suf¬ 
ferings of these useful and laborious creatures. 
o 
A physiological view of their effects will, perhaps, best justify 
their existence, and save them from such an imputation. 
To see the full purpose of their existence may not be within 
our power in our present state of knowledge, yet we nevertheless 
may introduce some remarks in this respect as a foundation, and 
whether a permanent one or otherwise, we shall not be tenacious 
about; they may, at least, lead to further consideration and 
research concerning them. 
Throughout nature the benevolent design appears of leaving 
no place idle or unoccupied, that can conveniently or usefully 
afford a home and happiness to some being suited to it, and which 
sometimes, in its turn, may also be made to contribute to the 
welfare of the animal it is given to infest. 
These parasite animals, though uncouth and almost of them¬ 
selves a disease, may prevent the access of worse diseases, and 
in such situations as would produce them of a fatal nature, and 
prevent it by their local effects. Children of cachectic habits 
breed worms faster than healthy children, which may tend to 
suppress or moderate the disease they incline to ; and sheep, in 
low damp situations, which might produce a worse disease, be¬ 
come in these situations particularly infested with these parasitic 
animals. Linnaeus, in respect to Pediculi or Lice, has admirably 
observed, “ Rodendo caput exciat achores apud puerulos voraces in- 
carceratos indeque stramosos et sicque preservat a coryza, tussi, coeci- 
tate, epilepsia” &c. That is, that Lice, by gnawing or irritating 
the skin of the head, excites a sort of running sores amongst boys 
