E THE COURSE OF EVOLUTION?

The question still remains: Does selection play any role in evolution, and,
if so, in what sense? Does the elimination of the unfit influence the
course of evolution, except in the negative sense of leaving more room for
the fit? There is something further to be said in this connection, although
opinions may differ as to whether the following interpretation of the term
"natural selection" is the only possible one.

[Illustration: FIG. 93. Evolution of elephant's skulls. (After Dendy.)]

If through a mutation a character appears that is neither advantageous nor
disadvantageous, but indifferent, the chance that it may become established
in the race is extremely small, although by good luck such a thing may
occur rarely. It makes no difference whether the character in question is a
dominant or a recessive one, the chance of its becoming established is
exactly the same. If through a mutation a character appears that has an
_injurious_ effect, however slight this may be, it has practically no
chance of becoming established.

[Illustration: FIG. 94. Evolution of elephant's trunk. (After Lull.)]

If through a mutation a character appears that has a _beneficial_ influence
on the individual, the chance that the individual will survive is
increased, not only for itself, but for all of its descendants that come to
inherit this character. It is this increase in the number of individuals
possessing a particular character, that might have an influence on the
course of evolution. This gives a better chance for improvement by several
successive steps; but not because the species is more likely to mutate
again in the same direction. An imaginary example will illustrate how this
happens: When elephants had trunks less than a foot long, the chance of
getting trunks more than one foot long was in proportion to the length of
trunks already present and to the number of individuals; but increment in
trunk length is no more likely to occur from an animal having a trunk more
than one foot long than from an animal with a shorter trunk.

The case is analogous to tossing pennies. At any stage in the game the
chance of accumulating a hundred heads is in proportion to the number of
heads already obtained, and to the number of throws still to be made. But
the number of heads obtained has no influence on the number of heads that
will appear in the next throw.

[Illustration: FIG. 95. Evolution of elephant's trunk: abov