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Another aspect to be considered is that unless environment evolve along with the organism the most promising new departures may be smothered by weeds.

Of supreme importance is the commonplace that man differs from the beasts that perish in having a lasting external heritage to which age after age contributes. There is social evolution as well as organic evolution, and social evolution has provided an apparatus whereby the gains of experience may swell the legacy of successive generations, although they do not, from the nature of the case, become part of the germinal inheritance. As Lloyd Morgan 1 well says: "The history of human progress has been mainly the history of man's higher educability, the products of which he has projected on to his environment. This educability remains, on the average, what it was a dozen generations ago; but the thought-woven tapestry of his surroundings is refashioned and improved by each succeeding generation."

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1

Few men have in greater measure enriched the thought-environment with which it is the aim of education to bring educable human beings into vital contact than has Charles Darwin."

1 "Mental Factors in Evolution," in "Darwinism and Modern Science" (1909), p. 445.

CHAPTER V

FACTS OF INHERITANCE

CHAPTER V

FACTS OF INHERITANCE

Progress during the Darwinian Era-Demonstration of Heritable
Qualities-Heredity, a Term for the Genetic Relation between
Successive Generations-Appreciation of Distinction between
Nature and Nurture-The Idea of the Continuity of Genera-
tions-Critical Attitude in Regard to Various Conclusions-
Mendelism-Methods of Studying Heredity-Microscopical
Study of the Germ-cells-Statistical Study: Filial Regression
-Galton's Law of Ancestral Inheritance-Experimental Study
-Pairing of Similar Pure-bred Forms-Blending-Particulate
Inheritance-Exclusive Inheritance-Reversion-New Depar-
tures-Mendelian Inheritance-Unit Characters-The Case
of Andalusian Fowls-Waltzing Mice-Occurrence of Mendelian
Inheritance-Practical Importance of Mendelism-Much

Progress but Great Uncertainty-Transmission of Acquired
Characters-Disease-Facts and Possibilities-A Striking Case
-Logical Position of the Question-Cases where the Theory
of Modification-inheritance is Inapplicable-Importance of
Environment and Function Remains-Selection and Stimulus
-Indirect Importance of Modifications-Practical Import of
the Question as to the Transmission of Acquired Characters-
Inheritance of Moral Character-Three General Conclusions.

EVEN in ancient times men pondered over the resemblances and differences between children and their parents, and wondered as to the nature of the bond which links generation to generation; but, although a recognition of these problems is old, the precise study of them is altogether modern, and may almost be called Darwinian. For it was largely through Darwin's influence that the scientific study of heredity began. "Before and after Darwin," Professor Osborn says, "will always be

the ante et post urbem conditam of biological history"; so it may be useful to inquire into the advances that have been made in the study of heredity since the beginning of Darwin's day.1

PROGRESS DURING THE DARWINIAN ERA. (1) Demonstration of Heritable Qualities.-Before 1859 much attention was given to the demonstration of the general fact of inheritance. In a large treatise like that of Prosper Lucas (1847) many hundreds of pages are devoted to proving, what we now take for granted-that our start in life is no haphazard affair, but rigorously determined by our parents and ancestors; that various peculiarities, important and trivial, useful and disadvantageous, reappear as part of the inheritance generation after generation.

This demonstration of heritability is still going on in reference to particular qualities; thus we have Prof. Karl Pearson's evidence in regard to such subtle qualities as longevity and fecundity, and his indirect proof that mental qualities illustrate the same law of inheritance as bodily qualities. It is very desirable that more data should be accumulated in regard to the heritability of variations, whether Darwin's "individual variations," or De Vries's mutations.' On the whole, however, it may be said that, since Darwin's day, sufficient evidence has been gathered to justify us in saying that any kind of character which appears as an inborn feature in an organism may be transmitted to the next generation.

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(2) "Heredity" a Term for the Genetic Rela

1 See, for a detailed discussion of what is dealt with briefly in this chapter, the author's treatise "Heredity" (2nd ed. Murray. London, 1912).

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