io6 NOTES OF A NATURALIST. 



mon to the north temperate zone are present in about 

 an equal proportion, while the representatives of the 

 tropical flora are but very few. 



With reference to the opinion expressed by writers 

 of authority, and especially by Engler,* that the 

 Andean flora is exceptionally rich in endemic genera 

 and species, and to the explanation which would 

 account for the facts, first, by the greater facility 

 afforded for the extension of new varieties in dry 

 climates, where the soil is not continuously covered 

 by the existing vegetation ; and, secondly, by the 

 isolation of the summits, favouring the development 

 of special local forms, I may venture on some sceptical 

 remarks. 



When we are struck by the large number of genera 

 and species that are exclusively confined to the 

 Andean flora, we are apt to forget the vast extent 

 of the region which we are contemplating. Even if 

 we exclude the mountains of Central America, and 

 also those of Southern Chili, from Araucania to the 

 Straits of Magellan, we have in the Andes a mountain 

 region considerably more than three thousand miles 

 in length, and from two hundred to over five hundred 

 miles in breadth. This vast region is as yet far from 

 being sufficiently explored to enable us to fix the 

 geographical limits of its genera and species with any 

 precision ; but it appears to me that, while a very 

 large number of genera are limited to the Andes as 

 a whole region, the range of most of them within the 

 limits of that region is very wide. I am further dis- 

 posed to form a similar opinion as to the distribution 



* " Versuch einer Entwicldungsgeschichte der Pflanzenwelt. " 



