It sometimes happens that the inter-relation established between one
living creature and another works in a retrograde direction. This is the
case with many thoroughgoing internal parasites which have sunk into an
easygoing kind of life, utterly dependent on their host for food,
requiring no exertions, running no risks, and receiving no spur to
effort. Thus we see that evolution is not necessarily progressive;
everything depends on the conditions in reference to which the living
creatures have been evolved. When the conditions are too easygoing, the
animal may be thoroughly well adapted to them--as a tapeworm certainly
is--but it slips down the rungs of the ladder of evolution.
This is an interesting minor chapter in the story of evolution--the
establishment of different kinds of parasites, casual and constant,
temporary and lifelong, external hangers-on and internal unpaying
boarders, those that live in the food-canal and depend on the host's
food and those that inhabit the blood or the tissues and find their food
there. It seems clear that ichneumon grubs and the like which hatch
inside a caterpillar and eat it alive are not so much parasites as
"beasts of prey" working from within.
But there are two sides to this minor chapter: there is the evolution of
the parasite, and there is also the evolution of counteractive measures
on the part of the host. Thus there is the maintenance of a bodyguard of
wandering amoeboid cells, which tackle the microbes invading the body
and often succeed in overpowering and digesting them. Thus, again, there
is the protective capacity the blood has of making antagonistic
substances or "anti-bodies" which counteract poisons, including the
poisons which the intruding parasites often make.