Chap. XI. DURING THE GLACIAL PERIOD. 379 



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 themselves 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 modifications in their structure, habits, and con- 

 stitutions 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 in 

 greater numbers, and having consequently been ad- 

 vanced through natural selection and competition to a 

 higher stage of perfection or dominating power, than the 

 southern forms. And thus, when they became com- 

 mingled during the Glacial period, the northern forms 

 were enabled to beat the less powerful southern forms. 

 Just in the same manner as we see at the present day, 



