freshwater fish called
the bitterling cannot continue its race without the unconscious
co-operation of the mussel. There are numerous mutually beneficial
partnerships between different kinds of creatures, and other
inter-relations where the benefit is one-sided, as in the case of
insects that make galls on plants. There are also among kindred animals
many forms of colonies, communities, and societies. Nutritive chains
bind long series of animals together, the cod feeding on the whelk, the
whelk on the worm, the worm on the organic dust of the sea. There is a
system of successive incarnations and matter is continually passing from
one embodiment to another. These instances must suffice to illustrate
the central biological idea of the web of life, the interlinked System
of Animate Nature. Linnaeus spoke of the Systema Naturae, meaning the
orderly hierarchy of classes, orders, families, genera, and species; but
we owe to Darwin in particular some knowledge of a more dynamic Systema
Naturae, the network of vital inter-relations. This has become more and
more complex as evolution has continued, and man's web is most complex
of all. It means making Animate Nature more of a unity; it means an
external method of registering steps of progress; it means an evolving
set of sieves by which new variations are sifted, and living creatures
are kept from slipping down the steep ladder of evolution.

Parasitism

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.