Having now obtained some insight into the factors concerned in disease, let us enquire further into the bearing of variation on these. It is evident that pathological conditions may vary; indeed they are themselves symptoms of variation, as we have seen. The history of all our cultivated plants shows abundantly that many of the variations obtained by breeding in our gardens, orchards, fields, etc., involve differences of response on the part of the plant to the very agencies which induce disease. Every year the florists' catalogues offer new "hardy" varieties; but a hardy variety is simply, for our present purpose, one which succumbs less readily to frost, cutting winds, cold damp weather, and so forth. If anyone doubts that hardy varieties have been gradually bred by selection, I refer him to the evidence collected by De Candolle, Darwin, Wallace, Bailey and others. When we come to enquire into the causes of "hardiness," however, difficulties at once beset us. The adaptation may express itself in a difference in the time of flowering or leafing, the exigencies of the season being "dodged," as it were, in a manner which was impossible with the original stock, as appears to have occurred with Peaches in America; or it may be expressed in deeper rooting, as is said to be the case in some Apples, or in the acquirement of a more deciduous habit, or in actually increased resistance to low temperatures. In such cases we cannot trace what alterations have occurred in the cells and tissues concerned, though we may be sure that some changes do occur. No experienced cultivator doubts that some varieties of Potato, Wheat, Vine, Chrysanthemum, etc., suffer more from epidemic diseases than others, and our yearly catalogues furnish us with plenty of promises of "disease-proof" varieties. Here also we may imagine several ways in which a particular variety may resist or escape the epidemic attacks of fungi which in the same neighbourhood decimate other varieties. If we could breed a variety of the Larch which opened its buds later than the ordinary form in our northern plains, the probability of its escaping the Larch-disease would be increased in proportion to the shortness of the period of tender foliation described on p. 153. It has been claimed for certain varieties of Wheat that increased thickness of the cuticle and fewer stomata per square unit of surface have diminished the risk of infection by Rust fungi, and for certain varieties of Potato, that the thicker periderm of the tuber protects them against fungi in the soil. That certain thick-skinned Apples, Tomatoes, and Plums pack and store better than those with a more tender epidermis seems proved--that is to say, they suffer less from fungi which gain access through bruises and other wounds; but it cannot be said that any convincing proof is yet to hand explaining in detail why some races of wheat resist Rust, or why the roots of American Vines suffer less from _Phylloxera_ than others.