Chap. XII.] IN THE NORTH AND SOUTH. 167 



widely separated areas, and belonging to genera not now 

 found in the intermediate torrid zones. 



It is a remarkable fact strongly insisted on by 

 Hooker in regard to America, and by Alph. de Can- 

 dolle in regard to Australia, that many more identical or 

 slightly modified species have migrated from the north 

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

 however, a few southern forms on the mountains of 

 Borneo and Abyssinia. I suspect that this preponder- 

 ant migration from the north to the 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 advanced 

 through natural selection and competition to a higher 

 stage of perfection, or dominating power, than the 

 southern forms. And thus, when the two sets became 

 commingled in the equatorial regions, during the alter- 

 nations of the Glacial periods, the northern forms were 

 the more powerful and were able to hold their places 

 on the mountains, and afterwards to migrate southward 

 with the southern forms; but not so the southern in 

 regard to the northern forms. In the same manner at 

 the present day, we see that very many European pro- 

 ductions cover the ground in La Plata, New Zealand, 

 and to a lesser degree in Australia, and have beaten 

 the natives; whereas extremely few southern forms have 

 become naturalised in any part of the northern hemi- 

 sphere, though hides, wool, and other objects likely to 

 carry seeds have been largely imported into Europe 

 during the last two or three centuries from La Plata 

 and during the last forty or fifty years from Australia. 

 The Neilgherrie mountains in India, however, offer a 

 partial exception; for here, as I hear from Dr. Hooker, 



