Lake Chalco is also greatly reduced in size by railway fillings and irrigation works, to the great distress of the natives who have gained their living by fishing in its waters since long before the Spanish conquest.
Of the former, the best known are the lakes of the Valley of Mexico - Texcoco, Chalco, Xochimilco, Zumpango, Xaltocan and San Cristobal - which are probably the remains of a lake once occupying the whole valley.
The other lakes are wholly different in character and surroundings, especially Chalco and Xochimilco.
Duges's statement that there is a second species of Amblystoma, which is normal in its metamorphosis, near Mexico but at a higher altitude, which may explain Velasco's observation that regularly transforming Amblystomas occur near that city; and thirdly, he made a careful examination of the two lakes, Chalco and Xochimilco, where the axolotls occur in abundance and are procured for the market.
The general elevation of the Valley is about 7500 ft., that of Lake Zumpango being 7493 ft., and of Lake Chalco 7480 ft.