Not infrequently the potato is seriously injured by blights which attack the leaves. The early blight, which usually appears in June, may destroy some of the foliage, thereby checking growth. The late blight, which also attacks the foliage, is far more serious. It differs little in outward appearance from the early blight. In rare cases the vines are so seriously injured that no potatoes are formed. The potato rot or blight did great damage to the potato in many localities in the United States in 1845. In 1846 the blight appeared in Ireland and virtually destroyed the entire crop. Before this date the potato had become the chief food supply of the peasantry. The cultivation of oats as a food crop had been universal before the introduction of the potato, but oats furnished so little food on a given area as compared to the potato that the cultivation of them at the time the blight appeared had been very largely abandoned. The loss of the potato crop produced widespread famine. The most conservative estimate of the numbers who perished for want of food or by disease caused by a meager diet of unhealthy and innutritions food is set down at six hundred thousand during the two years of the potato blight. This disease was not so destructive in 1847 as in 1846; and by 1848 it had virtually disappeared. Some one has said that if Great Britain had expended one dollar for investigating the diseases of potatoes where she had spent a thousand dollars for perfecting the engines of war, the terrible famine might have been averted. We now think it a relatively easy matter to keep the blight in check by thorough spraying with Bordeaux mixture. HOW THE POTATO HAS BEEN IMPROVED. All plants have their origin in pre-existing plants. While the young plant is always similar to the one from which it was derived, it is never exactly like its parent in every detail. This arises from the fact that all of the conditions under which the parent plant and its offspring grow are never exactly alike. The variations or differences in the plants are usually exceedingly small in a single generation; but occasionally they are wide, in which case they are called "sports" and are usually difficult to perpetuate. If successive generations of plants are reared under continuously improved conditions, there will be a continuous and accumulating variation from generation to generation, which in time may come to be so great as to make it difficult to discover a marked similarity between the wild and the cultivated forms of the same plant.