Alfred Russel Wallace 



developed aggressive plants have been artificially introduced. 

 Under nature it is these very aggressive species that would 

 first reach any island in their vicinity, and, being adapted 

 to the island and colonising it thoroughly, would then hold 

 their own against other plants from the same country, mostly 

 less aggressive in character. I have not explained this so 

 fully as I should have done in the book. Your criticism is 

 therefore useful. 



My Chap. XXIII. is no doubt very speculative, and I 

 cannot wonder at your hesitating at accepting my views. 

 To me, however, your theory of hosts of existing species 

 migrating over the tropical lowlands from the North Tem- 

 perate to the South Temperate zone appears more specula- 

 tive and more improbable. For, where could the rich 

 lowland equatorial flora have existed during a period of 

 general refrigeration sufficient for this ? and what became 

 of the wonderfully rich Cape flora which, if the tempera- 

 ture of Tropical Africa had been so recently lowered, would 

 certainly have spread northwards and on the return of the 

 heat could hardly have been driven back into the sharply 

 defined and very restricted area in which it now exists ? 



As to the migration of plants from mountain to moun- 

 tain not being so probable as to remote islands, I think 

 that is fully counterbalanced by two considerations : 



(a) The area and abundance of the mountain stations 

 along such a range as the Andes are immensely greater 

 than those of the islands in the North Atlantic, for example. 



(6) The temporary occupation of mountain stations by 

 migrating plants (which I think I have shown to be prob- 

 able) renders time a much more important element in in- 

 creasing the number and variety of the plants so dispersed 

 than in the case of islands, where the flora soon acquires 

 a fixed and endemic character, and where the number of 

 species is necessarily limited. 



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