344 THE ORIGIN OF SPECIES 



great glacial period occurred about 240,000 years ago, and en- 

 dured, 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 the most important result for us, 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 con- 

 versely it will be with the northern hemisphere, while the southern 

 passes through a glacial period. This conclusion throws so much 

 light on geographical distribution, that I am strongly inclined to 

 trust in it: but I will first give the facts which demand an ex- 

 planation. 



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, enor- 

 mously 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 belong- 

 ing to genera characteristic of the Cordillera. 



In Africa, several forms characteristic of Europe, and some few 

 representatives of the flora of the Cape of Good Hope, occur on 

 the mountains of Abyssinia. At the Cape of Good Hope a very few 

 European species, believed not to have been introduced by man, 

 and on the mountains several representative European forms, are 

 found which have not been discovered in the inter-tropical parts 

 of Africa. Dr. Hooker has also lately shown that several of the 

 plants living on the upper parts of the lofty island of Fernando 

 Po, and on the neighboring Cameroon Mountains, in the Gulf of 

 Guinea, are closely related to those on the mountains of Abyssinia, 

 and likewise to those of temperate Europe. It now also appears, 

 as I hear from Dr. Hooker, that some of these same temperate 

 plants have been discovered by the Rev. R. T. Lowe on the moun- 

 tains of the Cape Verde Islands. This extension of the same tem- 

 perate forms, almost under the equator, across the whole continent 

 of Africa and to the mountains of the Cape Verde archipelago, is 



