316 ALTERNATE GLACIAL PERIODS [CHAP. XII. 



these at long intervals are extremely severe, owing to certain 

 contingencies, of which the most important, as Sir C. Lyell has 

 shown, is the relative position of the land and water. 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 the most important 

 result for us, arrived at by Mr. Croll, is that whenever the 

 northern hemisphere passes through a cold period the tempera- 

 ture 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 will be with 

 the northern hemisphere, whilst the southern passes through a 

 Glacial period. This conclusion throws so much light on geo- 

 graphical distribution that I am strongly inclined to trust in it; 

 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 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 intertropical 

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

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

 Fernando Po and on the neighbouring 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 mountains of the Cape Verde islands. This extension of the 

 same temperate forms, almost under the equator, across the whole 

 continent of Africa and to the mountains of the Cape Verde 



