THE ANIMAL PARASITES 653 
tion to what we have already said about periproven- 
tricular filaridae in our Garden, indicating that the same 
inf estment probably also exists in London. 
A point brought out by Plimmer is to the effect that, 
of the several blood parasites, the microfilariae were the 
least harmful, and that of these the adult forms were the 
only ones to produce symptoms; yet in one place(38) he 
records microfilaria as plugging the cerebral capillaries 
of birds. This is a very important lesion if permanent, 
and especially so when affecting cerebral capillaries as 
do the organisms and pigment of malaria. The adult 
forms were fomid in one-fourth of the cases where micro- 
filaria were demonstrated. 
As to the pathogenicity of these blood parasites in 
general, it will be unsafe to arrive at a definite conclusion, 
recalling the pitfalls that I have already outlined in dis- 
cussing pathogenicity of parasites in general. Keeping 
in mind the wonderful adaptability on occasion of animals 
to unfavorable circumstances we must hesitate to declare 
unqualifiedly the importance of even blood parasites as 
morbid agents. Where the parasite is known to destroy 
the blood cells of birds and mammals it is otherwise, but 
even here experimental work would be necessary to settle 
the question. The element of ' ' racial ' ' immunity and of 
phylogeny is the fly in the ointment of our deductions. 
Transmission of Animal Parasitism From Wild Ani- 
mals TO Man. 
Examples of direct transmission will be only occa- 
sional, due to the relatively infrequent contacts between 
the two hosts. Pets threaten the most. Several such 
examples have been touched upon in the preceding pages 
and it but remains to gather them into one place. There 
is one concrete instance in the form of clear-cut simian 
scabies being transmitted to a keeper in this Garden (39) 
(38) Proc. Zool. 8oc. London, 1910, p. 134. 
(39) Weidman (F. D.), "Dermatoses of Monkeys," Arch. Derm, and 
Syph., Chicago, March, 1923, p. 289. 
