276 EVOLUTION AND DISEASE. 
by a series of well-planned experiments, has demon- 
strated that this insect acts as the intermediate host of 
the parasite. During the following four or six days the 
embryos complete their metamorphosis, and the friendly 
mosquito has completed its life cycle and dies, and its 
body probably falls into the water in which the eggs were 
deposited. The filaria by this time has developed so far 
that it is capable of living independently of the mosquito, 
and it seems probable that it remains in the water until 
it is captured by some animal in search of food, or 
swallowed by man, thus enabling it to complete its 
development. 
These facts indicate conclusively that this disease 
is spread and, in all probability, maintained by mos- 
quitoes, and the geographical distribution of the disease 
is coincident with that of culex. In this connection it is 
a fact of some importance to remember that filariae occur 
in other animals as well as in man. Manson states that 
half the dogs in China, all the magpies, one-third of the 
crows, and many other birds harbour similar hamatozoa 
in prodigious numbers ; and in South China where the 
filarial disease is endemic, if the blood of one thousand 
natives, selected indiscriminately, be examined some 
time between sunset and sunrise, in about one hundred 
the /ilaria sanguinis hominis will be discovered. 
The point of interest to us in connection with this 
disease is that though the filariz occur in dogs, magpies, 
and I have detected them in the blood of macaque 
monkeys, the enormous enlargement of the legs and 
other parts of the body, which is one of the chief cha- 
racters of the disease in man, has not been recorded in 
