Some of the modifications which the silk-moth has undergone stand in correlation with each other. Thus the eggs of the moths which produce white 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.