tion they
are usually found to have some of the parts or organs, which in closely
related forms are well developed, atrophied or entirely wanting. As a
rule this is a distinct advantage rather than a disadvantage to the
parasite, for those parts or organs that are lost would be useless or
even in the way in its special mode of life.

Then, too, the parasite often gives up all its independence and becomes
wholly dependent on its host or hosts not only for its food but for its
dissemination from one animal to another, in order that the species may
not perish with the host. But in return for all this it has gained a
life of ease, free from most of the dangers that beset the more
independent animals, and is thus able to devote its whole time and
energy to development and the propagation of the species.

We are accustomed to group the parasites that we know into two classes,
as harmful or injurious and as harmless, the latter including all those
kinds that do not ordinarily affect our well-being in any way. But such
a classification is not always satisfactory or safe, for certain
organisms that to-day or under present conditions are not harmful may,
on account of a great increase in numbers or change of conditions,
become of prime importance to-morrow. An animal that is well and strong
may harbor large numbers of parasites which are living at the expense of
some of the host's food or energy or comfort, yet the loss is so small
that it is not noticed and the intruders, if they are thought of at all,
are classed as harmless. Or we may at times even look upon them as
beneficial in one way or another. "A reasonable amount of fleas is good
for a dog. They keep him from brooding on being a dog."

But should these parasites for some reason or other increase rapidly
they might work great harm to the host. Even David Harum would limit the
number of fleas on a dog. Or the animal might become weakened from some
cause so that the drain on its resources made by the parasites, even
though they did not increase in numbers, would materially affect it.

Perhaps the most serious way in which parasites that are usually
harmless may become of great importance is illustrated by their
introduction into new regions or, as is more often the case, by the
introduction of new hosts into the region where the parasites are found.
Under normal conditions the animals of a given region are usually immune
to the parasites of the same region. That is, the