340 ON THE ORIGIN OF SPECIES 



with strange luxuriance at the base of the Himalaya, as 

 graphically described by Hooker. 



Thus, as I believe, a considerable number of plants, a 

 few terrestrial animals, and some marine productions, 

 migrated during the Glacial period from the northern 

 and southern temperate zones into the intertropical 

 regions, and some even crossed the equator. As 

 the warmth returned, these temperate forms would 

 naturally ascend the higher mountains, being exter- 

 minated on the lowlands ; those which had not reached 

 the equator would re-migrate northward or southward 

 towards their former homes ; but the forms, chiefly 

 northern, which had crossed the equator, would travel 

 still further from their homes into the more temperate 

 latitudes of the opposite hemisphere. Although we 

 have reason to believe from geological evidence that 

 the whole body of arctic shells underwent scarcely any 

 modification during their long southern migration and 

 re-migration northward, the case may have been wholly 

 different with those intruding forms which settled them- 

 selves on the intertropical mountains, and in the 

 southern hemisphere. These being surrounded by 

 strangers will have had to compete with many new 

 forms of life ; and it is probable that selected modifica- 

 tions in their structure, habits, and constitutions will 

 have profited them. Thus many of these wanderers, 

 though still plainly related by inheritance to their 

 brethren of the northern or southern hemispheres, now 

 exist in their new homes as well-marked varieties or as 

 distinct species. 



It is a remarkable fact, strongly insisted on by 

 Hooker in regard to America, and by Alph. de Candolle 

 in regard to Australia, that many more identical plants 

 and allied forms have apparently migrated from the 

 north to the south, than in a reversed direction. We 

 see, however, a few southern vegetable forms on the 

 mountains of Borneo and Abyssinia. I suspect that 

 this preponderant migration from north to south is 

 due to the greater extent of land in the north, and to 

 the northern forms having existed in their own homes 



