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.

THE EVIDENCES OF EVOLUTION--HOW IT CAME ABOUT

Sec. 1

Progress in Evolution

There has often been slipping back and degeneracy in the course of
evolution, but the big fact is that there has been progress. For
millions of years Life has been slowly creeping upwards, and if we
compare the highest animals--Birds and Mammals--with their predecessors,
we must admit that they are more controlled, more masters of their
fate, with more mentality. Evolution is on the whole _integrative_; that
is to say, it makes against instability and disorder, and towards
harmony and progress. Even in the rise of Birds and Mammals we can
discern that the evolutionary process was making towards a fuller
embodiment or expression of what Man values most--control, freedom,
understanding, and love. The advance of animal life through the ages has
been chequered, but on the whole it has been an advance towards
increasing fullness, freedom, and fitness of life. In the study of this
advance--the central fact of Organic Evolution--there is assuredly much
for Man's instruction and much for his encouragement.

Evidences of Evolution

In all this, it may be said, the fact of evolution has been taken for
granted, but what are the evidences? Perhaps it should be frankly
answered that the idea of evolution, that the present is the child of
the past and the parent of the future, cannot be _proved_ as one may
prove the Law of Gravitation. All that can be done is to show that it is
a key--a way of looking at things--that fits the facts. There is no lock
that it does not open.

But if the facts that the evolution theory vividly interprets be called
the evidences of its validity, there is no lack of them. There is
_historical_ evidence; and what is mo