Pearson writes: “As to the problem of evolution itself we are learning
to see it under a new light. Natural selection, combined with sexual
selection [by which Pearson means segregation of certain types through
individual selection] and heredity, is actually at work changing types.
We have quantitative evidence of its effects in many directions.” Yes!
but no evidence that selection of this sort can do anything more than
keep up the type to the upper limit attained in each generation by
fluctuating variations. Pearson adds, “Variations do not occur
accidentally, or in isolated instances; autogamic and assortive mating
are realities, and the problem of the near future is not whether
Darwinism is a reality, but what is quantitively the rate at which it is
working and has worked.” This statement expresses no more than Pearson’s
conviction that the process of evolution has taken place by means of
selection. He ignores other possibilities, which if established may put
the whole question in a very different light.
Heredity and Continuous Variation
It has been to a certain extent assumed in the preceding pages that both
parents are alike, or, if different, that they have an equal influence
on the offspring. This may be true in many cases for certain
characteristics. Thus a son from a tall father and a short mother may be
intermediate in height, or if the father is white and the mother black,
the children are mulattoes. But other characters rarely or never blend.
In such cases the offspring is more like one or the other parent, in
which case the inheritance is said to be exclusive. Thus if one parent
has blue eyes and the other black, some of the children may have black
eyes and others blue. There are also cases of particular inheritance
where there may be patches of color, some like the color of one parent,
some like that of the other parent. The latter two kinds of inheritance
will be more especially considered in the subsequent part of this
chapter; for the present we are here chiefly concerned with blended
characters.
How much in such cases does each parent contribute to the offspring?
This has been expressed by Galton in his law of ancestral heredity. This
law takes into account not only the two parents, but also the four
grandparents, and the eight great-grandparents, etc. There will be 1024
in the tenth generation. These 1024 individuals may be taken as a fair
sample of the general population, provided there has not been much
interbreeding. Are we then to look upon the individual as the fused or
blended product of the population a few generations back? If this were
true, should we not expect to find all the individuals of a community
very much alike, except for the fluctuating variations close around the
mode?
Public-domain text, read in full here on John Shaqi.
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