"As regards these plants, the globe may be divided into _five_ grand regions--the region of _rice_, which may be said to support the greatest number of the human race; the region of _maize_; of _wheat_; of _rye_; and lastly, of _barley_ and _oats_. The first three are the most extensive, and maize has the greatest range of temperature. [Illustration: FIG. 71.--The Rye District.] "The grains extending farthest north in Europe are barley and oats. Rye is the next, and is the prevailing grain in Sweden and Norway, and all the lands bordering on the Baltic, the North of Germany, and part of Siberia. Wheat follows rye; it is cultivated in the middle and South of France, England, part of Scotland, part of Germany, Hungary, Crimea, and the Caucasus. We next come to a district where wheat still abounds, but no longer exclusively furnishes bread,--rice and maize becoming frequent. To this zone belong Portugal, Spain, part of France, Italy, and Greece, Persia, Northern India, Arabia, Egypt, the Canary Islands, &c. Wheat can be reared wherever the mean temperature of the whole year is not under 37° or 39° F., and the mean summer heat, for a period of at least three or four months, is above 55°. It succeeds best on the limits of the sub-tropical region. In the Scandinavian peninsula, the cultivation of barley extends to 70° N. latitude, rye to 67°, and oats to 65°. The cultivation of rice prevails in Eastern and Southern Asia, and it is a common article of subsistence in various countries bordering on the Mediterranean. Maize succeeds best in the hottest and dampest parts of tropical climates. It may be reared as far as 40° N. and S. latitude on the American continent, on the western side; while in Europe it can grow even to 50° or 52° of latitude. It is now cultivated in all regions in the tropical and temperate zones which are colonised by Europeans. Millet of different kinds is met with in the hottest parts of Africa, in the South of Europe, in Asia Minor, and in the East Indies."[80] Professor Houston furnishes the following table in illustration of the distribution of _wheat_ and _barley_. It also shows the mean temperature which they require:-- BARLEY. Winter Summer Annual Mean. Mean. Mean. N. Lat. 62-1/2° Faroe, 39° 51° 45° 70 Lapland, 22 46 33 67·30' Russia, 9 46 32 57·30' Siberia, 0 60 32 WHEAT. Winter Summer Annual Mean. Mean. Mean.