Animals P by: Geoffrey LaPage Published by ... - PSSurvival.com

AnimakF Parasitic in Man
doubtedly suffers injury, is injured so little, or fights back so
successfully, that the parasite makes little or no difference to
its life or health. It may be difficult, indeed, where we find
host and parasite thus living together in a state of what we
call tokranu of each other, to decide whether these are instances
of parasitism, or even of symbiosis.
Parasitism, however, can always be infallibly distinguished
by one fact. Parasites always injure their hosts. The harm
done may be very slight; it may be visible only with the aid of
a microscope; or other methods of investigation, such as serological
or biochemical reactions, may be required to show that
it is there: but it does exist; and it marks the dividing line be
tween parasitism and all other forms of association between
living things.
So far, however, we have considered parasitism only in relation
to symbiosis and commensalism. We have established
it as a particular way of getting food and can say that a parasite
is an organism, whetler it be a plant, a virus, or an animal,
that gets its food from the bodies of other organisms
which are called its hosts, and that, as it does this, it inflicts
a degree of injury on these hosts. Is this enough to define the
parasite ? Does it, someone will say at once, distinguish the
parasite from the predator? The tiger, it can be argued, and
the microscopic carnivores that range through the green and
quiet dells in our ponds and streams, these are getting their
food from the bodies of other animals and these, too, harm
their prey; they kill them, in fact, which is more than the parasite
usually does. Extending our view beyond the animal
world, we may consider the cow that crops our fields, the rabbit
that does our crops so much harm, the plants indeed, that
feed on insects and other prey, and decide that all these seem
to fulfil our definition of the parasite. How can they be distinguished
from the parasite?
The distinction has been well expressed by Elton ( 1936)
who has said that: ‘The difference between a carnivore and a
parasite is simply the difference between living on capital and
income.. . .’ The parasite, that is to say the typical parasite,
does not destroy the animals or plants on which it feeds; it
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