620 Studies in the Theory of Descent. 



the so-called rainy season brings but little rain 50 

 little in comparison with the more southern regions, 

 where the heavy tropical thunderstorms daily 

 deluge the earth with water. Mexico lies much 

 too northerly, and does not reach the zone of 

 calms, within which region these tropical rains 

 are met with." 



Thus, in the high degree of dryness of the air 

 lasting throughout the year, I do not doubt that 

 we have the chief cause why no Amblystomas 

 occur on these elevated plains ; they simply cannot 

 exist, and would become dried up if taken there, 

 supposing them not to be able to change their 

 mode of life and to take to the water. If there- 

 fore in former times Amblystomas inhabited 

 Mexico, the coming on of the existing climatic 

 conditions left them only the alternative of be- 

 coming extinct, or of again taking to the aquatic 

 life of their Ichthyodeous ancestors. That this 

 was not directly possible that the Amblystoma 

 form was not able to become aquatic without a 

 change of structure, is shown by the fact that 

 even in the Lake of Mexico no Amblystoma 

 occurs. A retreat to an aqueous existence could, 

 as it appears, only be effected by complete re- 

 version to the Ichthyodeous form, which then also 

 took place. 



But my hypothesis of the transformation of the 



60 See also the excellent work upon Mexico by Miihlen- 

 pfordt already quoted, vol. i., pp. 69 76. 



