Origin of Species Variorum

certain contingencies, is extremely severe, and lasts for a great length of time. Mr. Croll believes that the last great Glacial period occurred about 240,000 years ago, and endured with slight alterations of climate for about 160,000 years. With respect to more ancient Glacial periods, several geologists are convinced from direct evidence that such occurred during the Miocene and Eocene formations, not to mention still more ancient formations. But in relation to our present subject, the most important result arrived at by Mr. Croll is, that whenever the northern hemisphere passes through a cold period, the temperature of the southern hemisphere is actually raised, with the winters rendered much milder, chiefly through changes in the direction of the ocean-currents. So conversely it is with the northern hemisphere, when the southern passes through a glacial period. These conclusions have, as we shall immediately see, a most important bearing on geographical distribution; but I will first give the facts, which demand an explanation. ↑

In South America, Dr. Hooker has shown that besides many closely allied species, between forty and fifty of the flowering plants of Tierra del Fuego, forming no inconsiderable part of its scanty flora, are common to North America and Europe, enormously remote as these areas in opposite hemispheres are from each other. On the lofty mountains of equatorial America a host of peculiar species belonging to European genera occur. On the Organ mountains of Brazil, some few temperate European, some Antarctic, and some Andean genera were found by Gardner, which do not exist in the low intervening hot countries. On the Silla of Caraccas the illustrious Humboldt long ago found species belonging to genera characteristic of the Cordillera. In Africa, several forms characteristic of Europe and