And
probably when thrown on the world, as when nests are blown down, or the
birds get killed, or change their quarters, as they often do, it is able
to exist for some time without avian blood. Let us then imagine some of
these orphaned colonies, unable to find birds, but through a slight
change in habits or organization able to exist in the imago state
without sucking blood until they laid their eggs; and succeeding
generations, still better able to stand the altered conditions of life
until they become practically independent (like gnats), multiplying
greatly, and disporting themselves in clouds over forests, yet still
retaining the old hunger for blood and the power to draw it, and ready
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.
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