492 THE WRITING OF THE ' ORIGIN OF SPECIES.' [1S58. 



now is ; and then the temperate parts of Europe and America 

 would be separated, as far as migration is concerned, just 

 as they now are. Then came on the Glacial period, driv- 

 ing far south all living things ; middle or even southern Eu- 

 rope being peopled with Arctic productions ; as the warmth 

 returned, the Arctic productions slowly crawled up the 

 mountains 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' 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-doc- 

 trines, 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 con- 

 sideration ; there is a considerable body of geological evi- 

 dence 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 be- 

 lieve that at the height of the Glacial epoch, a72d when all 

 Tropical produciio7ts must have been considerably distressed^ 

 that 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 di- 

 rection northward. (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 temper- 

 ate intruders would crawl up the mountains. Hence the Eu- 

 ropean forms on Nilgherries, Ceylon, summit of Java, Organ 

 Mountains of Brazil. But these intruders beins: surrounded 



