COLOUR, CHEMICAL PROPERTIES, AND IN THE STATE OF
THE TISSUES--LOCAL DISEASES--CONSPICUOUS MODIFICATIONS FROM CHANGED
CLIMATE OR FOOD, ETC.--PLUMAGE OF BIRDS AFFECTED BY PECULIAR NUTRIMENT,
AND BY THE INOCULATION OF POISON--LAND-SHELLS--MODIFICATIONS OF ORGANIC
BEINGS IN A STATE OF NATURE THROUGH THE DEFINITE ACTION OF EXTERNAL
CONDITIONS--COMPARISON OF AMERICAN AND EUROPEAN TREES--GALLS--EFFECTS
OF PARASITIC FUNGI--CONSIDERATIONS OPPOSED TO THE BELIEF IN THE POTENT
INFLUENCE OF CHANGED EXTERNAL CONDITIONS--PARALLEL SERIES OF
VARIETIES--AMOUNT OF VARIATION DOES NOT CORRESPOND WITH THE DEGREE OF
CHANGE IN THE CONDITIONS--BUD-VARIATION--MONSTROSITIES PRODUCED BY
UNNATURAL TREATMENT--SUMMARY.

If we ask ourselves why this or that character has been modified under
domestication, we are, in most cases lost in utter darkness. Many
naturalists, especially of the French school, attribute every modification
to the "monde ambiant," that is, to changed climate, with all its
diversities of heat and cold, dampness and dryness, light and electricity,
to the nature of the soil, and to varied kinds and amount of food. By the
term definite action, as used in this chapter, I mean an action of such a
nature that, when many individuals of the same variety are exposed during
several generations to any change in their physical conditions of life,
all, or nearly all the individuals, are modified in the same manner. A new
sub-variety would thus be produced without the aid of selection.

I do not include under the term of definite action the effects of habit or
of the increased use and disuse of various organs. Modifications of this
nature, no doubt, are definitely caused by the conditions to which the
beings are subjected; but they depend much less on the nature of the
conditions than on the laws of growth; hence they are included under a
distinct head in the {272} following chapter. We know, however, far too
little of the causes and laws of variation to make a sound classification.
The direct action of the conditions of life, whether leading to definite or
indefinite results, is a totally distinct consideration from the effects of
natural selection; for natural selection depends on the survival under
various and complex circumstances of the best-fitted individuals, but has
no relation whatever to the primary cause of any modification of structure.

I will first give in detail all the facts which I have b