Some of the modifications which the silk-moth has undergone stand in correlation with one another. 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. (8/83. Quatrefages 'Etudes' etc. pages 12, 209, 214.) 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 (8/84. Robinet 'Manuel' etc. page 303.) 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. (8/85. Robinet ibid page 15.)]
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 a corresponding period. And finally we see that insects are amenable to the great principle of Selection.
CHAPTER 1.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 truly wild plants, so that there is no safe standard of comparison by which to judge of any supposed amount of change. Not a few botanists believe that several of our anciently cultivated plants have become so profoundly modified that it is not possible now to recognise their aboriginal parent-forms. Equally perplexing are the doubts whether some of them are descended from one species, or from several inextricably commingled by crossing and variation. Variations often pass into, and cannot be distinguished from, monstrosities; and monstrosities are of little significance for our purpose. Many varieties are propagated solely by grafts, buds, layers, bulbs, etc., and frequently it is not known how far their peculiarities can be transmitted by seminal generation. Nevertheless, some facts of value can be gleaned: and other facts will hereafter be incidentally given. One chief object in the two following chapters is to show how many characters in our cultivated plants have become variable.
Before entering on details a few general remarks on the origin of cultivated plants may be introduced. M. Alph. De Candolle (9/1. 'Geographie botanique raisonnee' 1855 pages 810 to 991.) in an admirable discussion on this subject, in which he displays a wonderful amount of knowledge, gives a list of 157 of the most useful cultivated plants. Of these he believes that 85 are almost certainly known in their wild state; but on this head other competent judges (9/2.