348 THE ORIGIN OF SPECIES 



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 the two sets became commingled in the equatorial 

 regions, during the alternations of the Glacial periods, the north- 

 ern forms were the more powerful and were able to hold their 

 places on the mountains, and afterward 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 productions 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 naturalized in any part of the northern hemisphere, though 

 hides, wool, and other objects likely to carry seeds have been 

 largely imported into Europe during the last two or three cen- 

 turies from La Plata, and during the last forty or fifty ^^ears from 

 Australia. The Neilgherrie Mountains in India, howei^er, offer a 

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

 forms are rapidly sowing themselves, and becomir-i naturalized. 

 Before the last great Glacial period, no doubt the inter-tropical 

 mountains were stocked with endemic alpine forms; but these 

 have almost everywhere yielded to the more dominant forms gen- 

 erated in the larger areas and more efficient workshops of the 

 north. In many islands the native productions are nearly equalled, 

 or even outnumbered, by those which have become naturalized; 

 and this is the first stage toward their extinction. Mountains are 

 islands on the land, and their inhabitants have yielded to those 

 produced within the larger areas of the north, just in the same 

 way as the inhabitants of real islands have everywhere yielded 

 and are still yielding to continental forms naturalized through 

 man's agency. 



The same principles apply to the distribution of terrestrial ani- 

 mals and of marine productions, in the northern and southern 

 temperate zones, and on the inter-tropical mountains. When, dur- 

 ing the height of the Glacial period, the ocean-currents were 

 widely different to what they now are, some of the inhabitants of 

 the temperate seas might have reached the equator; of these a 

 few would perhaps at once be able to migrate southward, by 

 keeping to the cooler currents, while others might remain and sur- 

 vive in the colder depths until the southern hemisphere was in 

 its turn subjected to a glacial climate and permitted their further 

 progress; in nearly the same manner as, according to Forbes, 



