136 THE WRITING OF THE 'ORIGIN OF SPECIES.' [1858. 



Europe being peopled with Arctic productions ; as the warmth 

 returned, the Arctic productions slowly crawled up the moun- 

 tains as they became denuded of snow ; and we now see on 

 their summits the remnants of a once continuous flora and 

 fauna. This is E. Forbes's theory, which, however, I may 

 add, I had written out four years before he published. 



Some facts have made me vaguely suspect that between 

 the glacial and the present temperature there was a period 

 of slightly greater warmth. According to my modification- 

 doctrines, I look at many of the species of North America 

 which closely represent those of Europe, as having become 

 modified since the Pliocene period, when in the northern part 

 of the world there was nearly free communication between 

 the old and new worlds. But now comes a more important 

 consideration ; there is a considerable body of geological 

 evidence that during the Glacial epoch the whole world was 

 colder ; I inferred that, many years ago, from erratic boulder 

 phenomena carefully observed by me on both the east and 

 west coast of South America. Now I am so bold as to 

 believe that at the height of the Glacial epoch, and when all 

 Tropical productions must have been considerably distressed, 

 several temperate forms slowly travelled into the heart of the 

 Tropics, and even reached the southern hemisphere; and some 

 few southern forms penetrated in a reverse direction north- 

 ward. (Heights of Borneo with Australian forms, Abyssinia 

 with Cape forms.) Wherever there was nearly continuous high 

 land, this migration would have been immensely facilitated ; 

 hence the European character of the plants of Tierra del Fuego 

 and summits of Cordilleras ; hence ditto on Himalaya. As the 

 temperature rose, all the temperate intruders would crawl up 

 the mountains. Hence the European forms on Nilgherries, 

 Ceylon, summit of Java, Organ Mountains of Brazil. But 

 these intruders being surrounded with new forms would be 

 very liable to be improved or modified by natural selection, 

 to adapt them to the new forms with which they had to 



