cocoons and of those which produce yellow cocoons differ slightly
in tint. The abdominal feet also of the caterpillars which yield white
cocoons are always white, whilst those which give yellow cocoons are
invariably yellow.[516] We have seen that the caterpillars with dark
tiger-like stripes produce moths which are more darkly shaded than
other moths. It seems well established[517] that in France the
caterpillars of the races which produce white silk, and certain black
caterpillars, have resisted, better than other races, the disease which
has recently devastated the silk-districts. Lastly, the races differ
constitutionally, for some do not succeed so well under a temperate
climate as others; and a damp soil does not equally injure all the
races.[518]

From these various facts we learn that silk-moths, like the higher animals,
vary greatly under long-continued domestication. We learn also the more
important fact that variations may occur at various periods of life, and be
inherited at corresponding periods. And finally we see that insects are
amenable to the great principle of Selection.

* * * * *

{305}

CHAPTER IX.

CULTIVATED PLANTS: CEREAL AND CULINARY PLANTS.

PRELIMINARY REMARKS ON THE NUMBER AND PARENTAGE OF CULTIVATED
PLANTS--FIRST STEPS IN CULTIVATION--GEOGRAPHICAL DISTRIBUTION OF
CULTIVATED PLANTS.

CEREALIA.--DOUBTS ON THE NUMBER OF SPECIES.--WHEAT: VARIETIES
OF--INDIVIDUAL VARIABILITY--CHANGED HABITS--SELECTION--ANCIENT HISTORY
OF THE VARIETIES.--MAIZE: GREAT VARIATION OF--DIRECT ACTION OF CLIMATE
ON.

CULINARY PLANTS.--CABBAGES: VARIETIES OF, IN FOLIAGE AND STEMS, BUT NOT
IN OTHER PARTS--PARENTAGE OF--OTHER SPECIES OF BRASSICA.--PEAS: AMOUNT
OF DIFFERENCE IN THE SEVERAL KINDS, CHIEFLY IN THE PODS AND SEED--SOME
VARIETIES CONSTANT, SOME HIGHLY VARIABLE--DO NOT
INTERCROSS.--BEANS.--POTATOES: NUMEROUS VARIETIES OF--DIFFERING LITTLE,
EXCEPT IN THE TUBERS--CHARACTERS INHERITED.

I shall not enter into so much detail on the variability of cultivated
plants, as in the case of domesticated animals. The subject is involved in
much difficulty. Botanists have generally neglected cultivated varieties,
as beneath their notice. In several cases the wild prototype is unknown or
doubtfully known; and in other cases it is hardly possible to distinguish
between escaped seedlings and t