Tlie Transformation of the Mexican Avolotl. 619 



Mexican plateau might not perhaps be due to the 

 same cause, i.e. whether such a dryness of the 

 atmosphere might not perhaps prevail also in that 

 region, so that Amphibia, or at least salamander- 

 like Amphibia, could not long exist on the land. 

 The height above the sea is still greater (7000 

 to 8000 feet), and the tropical sun would more 

 rapidly dessicate everything in a country poor in 

 water. 



As I was at the time without any books that 

 might have enlightened me on the meteorological 

 conditions of Mexico, I wrote to Dr. v. Frantzius, 

 who, by many years residence in Central America 

 was familiar with the climate of this region, and 

 solicited his opinion. I received the reply that 

 on the high plains of Mexico an extraordinary 

 dryness of the atmosphere certainly prevails. 

 " The main cause of the dryness of the high 

 plains is to 'be found in the geographical po- 

 sition, the configuration of the land, and the 

 physical structure. The north-eastern trade- 

 wind drives the clouds against the mountains, 

 on the summits of which they deposit their 

 moisture, so that no vapour is carried over; 

 as long as the north-east trade-wind blows, the 

 streams feeding the rivers flowing into the At- 

 lantic Ocean are abundantly fed with water, whilst 

 on the western slopes, and especially on the high 

 plains, the clouds give no precipitation. In the 

 second half of the year also, during our summer, 



S s 



