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In the last twenty-five years, however, an enormous amount of new
facts about evolution and heredity have been discovered, and the
balance has now swung over heavily, and, I think, permanently, in
favour of Darwinism or selectionism. Chief among these new facts
is the discovery that most mutations are not large, but very small
steps of change.
It turns out that the reports of the death of Darwinism, like those
of the death of Mark Twain, were very much exaggerated. Indeed,
the net result of the last quarter-century's work in biology has been
the re-establishment of natural selection as the essential method of
evolution, and its re-establishment not merely where Darwin left it,
but on a far more secure footing. For one thing, the alternative ex-
planations have ceased to be plausible. First among these is Lamarck-
ism, or the so-called inheritance of acquired characters (which means
the inheritance of characters acquired by an individual as a result of
changes in the environment, like tanning due to sun, or of use or dis-
use of organs, like the more powerful muscles of the athlete or heavy
worker; it docs not refer to characters "acquired" through new
mutation). This has now been thoroughly discredited. It has been
definitely disproved in a number of cases; it cannot in any case apply
to a large range of facts (such as the evolution of the hard skeleton
of higher insects, or of our own teeth); the apparent examples of its
existence have all been shown cither to be due to error or susceptible
of an alternative explanation; and it is logically self-contradictory.
Second, there is orthogenesis, or evolution in a predetermined
direction, supposedly due to the germ-plasm being predestined to
vary only in a certain way. It is true that when we can trace the
actual course of evolution by means of abundant fossils, we often find
that it docs proceed in straight lines. The most familiar example is
the steady evolution of the horse toward speed and the one-toed foot
and toward elaborate teeth for grinding grassóbut wherever (as is
in most cases obvious) the direction is toward greater efficiency, this
is to be expected on the basis of natural selection. In any case, there
are some examples, like that of the elephants or the baboons, where
evolution is not in a straight line, but changes direction during its
course. There arc a few puzzling cases, like the trend toward appar-
ently useless or harmful characters, as seen in a number of groups of
Ammonites shortly before their final extinction; but they arc quite
exceptional, and may prove to be susceptible of alternative explana-
tion. In any case, orthogenesis in a useless (or harmful) direction
would demand mutation-rates much higher than any yet found in