Chapter III: 52

Variation

The assumption of Darwin and Wallace that variations are haphazard
in origin and indefinite in direction—If these assumptions be not
correct Natural Selection ceases to be the fundamental factor in
evolution—Darwin’s views regarding variation underwent
modification—He eventually recognised the distinction between
definite and indefinite variations, and between continuous and
discontinuous variations—Darwin attached but little importance to
either definite or discontinuous variations—Darwin’s views on the
causes of variations—Criticism of Darwin’s views—Variations appear
to occur along certain definite lines—There seems to be a limit to
the extent to which fluctuating variations can be accumulated—De
Vries’ experiments—Bateson on “discontinuous variation”—Views held
by De Vries—Distinction between continuous and discontinuous
variations—The work of De Vries—Advantages enjoyed by the botanist
in experimenting on the making of species—Difficulties encountered
by the animal breeder—Mutations among animals—The distinction
between germinal and somatic variations—The latter, though not
transmitted to offspring, are often of considerable value to their
possessor in the struggle for existence.

CHAPTER IV 111

Hybridism

The alleged sterility of hybrids a stumbling-block to
evolutionists—Huxley’s views—Wallace on the sterility of
hybrids—Darwin on the same—Wallace’s theory that the infertility of
hybrids has been caused by Natural Selection so as to prevent the
evils of intercrossing—Crosses between distinct species not
necessarily infertile—Fertile crosses between species of
plants—Sterile plant hybrids—Fertile mammalian hybrids—Fertile bird
hybrids—Fertile hybrids among amphibia—Limits of
hybridisation—Multiple hybrids—Characters of hybrids—Hybridism does
not appear to have exercised much effect on the origin of new
species.

CHAPTER V 133

Inheritance

Phenomena which a complete theory of inheritance must explain—In
the present state of our knowledge it is not possible to formulate
a complete theory of inheritance—Different kinds of
inheritance—Mendel’s experiments and theory—The value and
importance of Mendelism has been exaggerated—Dominance sometimes
imperfect—Behaviour of the nucleus of the sexual
cell—Chromosomes—Experiments of Delage and Loeb—Those of Cuénot on
mice and Castle on guinea pigs—Suggested modification of the
generally-accepted Mendelian formulæ—Unit characters—Biological
isomerism—Biological molecules—Interpretation of the phenomena of
variation and heredity on the conception of biological
molecules—Correlation—Summary of the conception of biological
molecules.