There is another side of evolution so obvious that it is often
overlooked, the tendency to link lives together in vital
inter-relations. Thus flowers and their insect visitors are often
vitally interlinked in mutual dependence. Many birds feed on berries and
distribute the seeds. The tiny freshwater snail is the host of the
juvenile stages of the liver-fluke of the sheep. The mosquito is the
vehicle of malaria from man to man, and the tse-tse fly spreads sleeping
sickness. The freshwater mussel cannot continue its race without the
unconscious co-operation of the minnow, and the 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. Linnæus spoke of the Systema Naturæ, 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
Naturæ, 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.