The connection between the flea and the mammal it feeds on is even less
close than that which exists between the Ornithomyia and bird. The fact
that fleas are so common and universal--for in all lands we have them,
like the poor, always with us; and that they are found on all mammals,
from the king of beasts to the small modest mouse--seems to show a great
amount of variability and adaptiveness, as well as a very high
antiquity. It has often been reported that fleas have been found hopping
on the ground in desert places, where they could not have been dropped
by man or beast; and it has been assumed that these "independent" fleas
must, like gnats and ticks, subsist on vegetable juices. There is no
doubt that they are able to exist and propagate for one or two years
after being deprived of their proper aliment; houses shut up for a year
or longer are sometimes found infested with them; possibly in the
absence of "vegetable juices" they flourish on dust. I have never
detected them hopping on the ground in uninhabited places, although I
once found them in Patagonia, in a hamlet which had been attacked and
depopulated by the Indians about twenty months before my visit. On
entering one of the deserted huts I found the floor literally swarming
with fleas, and in less than ten seconds my legs, to the height of my
knees, were almost black with their numbers. This proves that they are
able toincrease greatly for a period without blood; but I doubt that
they can go on existing and increasing for an indefinite time; perhaps
their true position, with regard to the parasitical habit, is midway
between that of the strict parasite which never leaves the body, and
that of independent parasites like the Culex and the Ixodes, and all
those which are able to exist free for ever, and are parasitical only
when the opportunity offers.
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