n terms of Differential
Evolution,[23] somewhat thus:

(1) The first, the least speculative, evolutionary criterion of an
animal is its degree of adaptation to its environment.

(2) Man exhibits a less degree of adaptation to environment than
any other animal; principally because (_a_) he consists, roughly
speaking, incomparably more than any other animal, of three
interdependent parts--body, thinking brain, and that higher mental
function that we call spirit--the development of any one of which,
beyond a certain stage, is found to be detrimental to the other
two; and because (_b_) he is able possibly to control directly his
own evolution, and certainly to modify it indirectly by modifying
the environment in which he evolves. He is able to make mistakes in
his own evolution.

(3) The typical poor man is better adapted to his environment, such
as it is, than the typical man of any other class; for he has been
kept in closer contact with the primary realities--birth, death,
risk, starvation;--in closer contact, that is to say, with those
sections of human environment which are not of human making and
which are common to all classes. He has fewer mistakes to go back
upon.

[23] Evolution is at present the last refuge of unscientific
minds which think they have explained a process when they
have given it a new name, just as chemists used to call an
obscure chemical action _catalytic_ and then assume that its
nature was plain. _Evolution_ means an _unfolding_. In that
sense it is an observed fact, though exactly how the
unfolding is brought about is still conjectural. But it does
not matter for the purposes of my argument whether human
beings evolve by the transmission to offspring of acquired
characteristics, or by bequeathing to them as birthright an
environment that their fathers had to make. The material for
constructing any theory of mental, or joint mental and
physical evolution, is so hazy that one cannot do more than
speculate. It may be noted, however, that acquired mental
characteristics appear to be more transmissible, and less
stable, than acquired physical characteristics; and that
mental evolution (in the broad sense again) proceeds faster
and collapses m