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There are also the vitalistic theories of a mysterious life-force or
unconscious purpose, like Bergson's elan vital. However, these are in
reality not explanations at all, but mere confessions of ignorance.
To say that life evolves because of an elan vital is on a par with saying
that a locomotive runs because of an elan locomotif.
Not only have the alternative explanations become implausible,
but a great deal of new support has been forthcoming for the theory
of natural selection. One of Darwin's difficulties about his own theory
(which caused him to give greater weight to Lamarckism than he
would otherwise have done) was that he could not sec how new
hereditary variations of small extent—what we to-day should call
small mutations—could be preserved and kept from being swamped
by crossing. This, as R. A. Fisher has pointed out, was due to his
acceptance of the idea, current in his time, of " blending inheritance."
In a cross between two distinct types, the material bases of their
heredity (and Darwin's generation completely lacked concrete know-
ledge on this subject) were supposed to blend in the resultant off-
spring, as two drops of coloured ink will blend with each other.
Thus, any new character would be quite literally diluted on cross-
ing with the original type, and would soon fade out. The essence of
Mendelism, however, is that the genes or units of heredity remain
unchanged (apart from rare mutation), however they arc combined
with other genes. Many of the new genes produced by mutation can
remain in the germ-plasm indefinitely until conditions are favourable,
when they will begin to increase their representation in the stock.
If a new mutant gene is recessive—i.e. must appear in double dose
before it produces any visible effect—it can be carried in single dose
for an indefinite period, even if it is slightly deleterious.
What is more, we now know that the effects of genes can be
markedly altered by other genes, and numerous examples exist where
slightly deleterious genes have been rendered harmless or even bene-
ficial by being "buffered," in the chemist's phraseology, by new com-
binations of other genes. A beautiful example comes from domestic
dogs. In producing the show type of St. Bernard, man has encouraged
features characteristic of abnormal overgrowth of the pituitary gland:
yet St. Bernards are not themselves abnormal, as a man with com-
parable characteristics would be. However, when St. Bernards arc
crossed with other breeds like Great Danes, a considerable number
of the offspring show actual pathological symptoms. In producing
his ideal of a -St. Bernard, man has selected for genes making the
pituitary abnormal: but he has also aimed at healthy dogs and so
has automatically selected for other genes which would prevent the