It amounts to this, therefore, that in the plains the long continued period of foliation allows insects, frost, winds, etc., some six weeks or two months in which to injure the slowly sprouting tender shoots, whereas in the mountain heights they have only a fortnight or so in which to do such damage. That the lower altitude and longer summer are not in themselves inimical to Larch is proved by the splendid growths made by the trees first planted a century ago. Then came the epidemic of Larch-disease: the fungus, which is merely endemic--_i.e._ obtains a livelihood here and there on odd trees, or groups of trees in warmer or damper nooks--in the Alps, was favoured by the more numerous points of attack afforded to its spores by injuries due to insects--_Coleophora_, _Chermes_, etc.--and frost wounds, as well as by the longer periods of moist dull weather, and the longer season of foliation. Moreover, as time went on almost every consignment of young Larch-trees sent abroad was already infected. Here again, then, we find the factors of an epidemic consisting in events which favour the reproduction and spread of a fungus more than they do the well-being of the host. As a third illustration I will take the case of an insect epidemic. In 1863 a disease was observed on vines in the South of France which frightened the growers as they realised its destructive effects: the roots decayed and the leaves turned yellow and died before the grapes ripened, and such vines threw out fewer and feebler shoots the following year, and often none at all afterwards. In 1865 the disease was evidently becoming epidemic near Bordeaux, and in 1868 it was shown to be due to an insect, _Phylloxera_, the female of which lays its eggs on the roots, where they hatch. The louse-like offspring sticks its proboscis into the tissues as far as the central cylinder. The irritated pericycle and cortex then grow and form nodules of soft juicy root-tissue at which the insect continues to suck. Rapid reproduction results in the majority of the young rootlets being thus attacked, and since they cannot form their normal periderm and harden off properly they rot, and admit fungi and other evils, in consequence of which the vine suffers also in the parts above ground. Evidence that the general damage is due to the diminished root-action is found in the peculiarly dry poor wood formed in the "canes" of diseased plants. By 1877 the epidemic had spread to the northern limits of the French vineyards, and by 1888 half the vines in the country were attacked, and the yield of wine reduced from half a million hectolitres to 50,000 only. Meanwhile the disease had spread to Italy, Germany, Madeira, Portugal, and even to the Cape, though not in epidemic form as in the Bordeaux centre whence it spread.