Chap. XI. 
DURING THE GLACIAL PERIOD. 
379 
lands; 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 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, ^d 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 
