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.

Entomologists regard the flea as a degraded fly. Certainly it is very
much more degraded than the bird-borne Ornithomyia, with its subtle
motions and instinct, its power of flight and social pastimes. The poor
pulex has lost every trace of wings; nevertheless, in its fallen
condition it has developed some remarkable qualities and saltatory
powers, which give it a lower kind of glory; and, compared with another
parasite with which it shares the human species, it is almost a noble
insect. Darwin has some remarks about the smallness of the brain of an
ant, assuming that this insect possesses a very high intelligence, but I
doubt very much that the ant, which moves in a groove, is mentally the
superior of the unsocial flea. The last is certainly the most teachable;
and if fleas were generally domesticated and made pets of, probably
there would be as many stories about their marvellous intelligence and
fidelity to man as we now hear about our over-praised "friend" the dog.

With regard to size, the flea probably started on its downward course as
a comparatively large insect, probably larger than the Ornithomyia. That
insect has been able to maintain its existence, without dwindling like
the Leptus into a mere speck, through the great modification in organs
and instinct, which adapt it so beautifully to the feathery element in