at any moment to return to the ancestral habit. It might be said that if
such a result were possible it would have occurred, but that we find no
insect like the Ornithomyia existing independently. With the bird-fly it
has not occurred, as far as we know; but in the past history of some
independent parasites it is possible that something similar to the
imaginary case I have sketched may have taken place. The bush-tick is a
more highly specialized, certainly a more degraded, creature than the
bird-fly, and the very fact of its existence seems to show that it is
possible for even the lowest of the fallen race of parasites to start
afresh in life under new conditions, and to reascend in the scale of
being, although still bearing about it the marks of former degeneracy.

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