218 AMPHIBIA 



shores unless the water was also saline. About eight miles to 

 the south-east of the city is the lake of Xochimilco and just 

 beyond it another called Chalco. The level of these lakes is 

 about ten feet higher than that of Texcoco, and they are filled 

 with perfectly fresh water and surrounded by fertile meadows. 

 Xochimilco is broken up by hundreds of small islands of 

 various sizes separated by channels and canals ; these are 

 known as chinamps or floating gardens although they are not 

 floating but rise from the bottom. Many of them are made 

 artificially, floating masses of vegetation being fixed by stakes 

 of willow and poplar which soon take root and grow, and then 

 mud is ladled up from the bottom and spread on the mass 

 until an island is made sufficiently firm for cultivation. The 

 water teems with both vegetable and animal life, it is 

 full of decomposing vegetable matter and swarms with insect 

 larvae, worms and the celebrated axolotls." 



Dr. Gadow's conclusion concerning the reason for the 

 axolotl remaining in the larval condition is as follows : " The 

 unfailing abundance of food and water, the innumerable hiding- 

 places for them in the mud, under the banks, and amongst the 

 reeds, all these features are attractions so great that the creatures 

 remain in their paradise, and consequently retain all those larval 

 characters which are not directly connected with propagation. 

 There is nothing to prevent them from leaving the lake and 

 becoming land newts, but there is nothing also to induce 

 them to do so." This does not seem to me altogether convinc- 

 ing or scientific. It is true that abundance of food may be one 

 of the conditions which favours the retention of the larval organs 

 and tends to postpone the metamorphosis, but the important 

 point to consider is not that the animals remain in the water 

 but that they are able to continue to breathe in the water. 

 There are many Amphibia which are aquatic in the adult con- 

 dition, for example Pipa and Dactylethra among the Anura and 

 Triton waltlii among the Urodela, and yet pass through the 

 metamorphosis as usual and breathe air in the adult condition, 

 rising to the surface to obtain it. Dr. Gadow himself proceeds to 

 inform us that in the mountain streams of the Sierra de Ajusco, 

 not far from the lake where the axolotls flourish, lives another 

 species of Amblystoma, A. altamirani, which passes through the 

 normal metamorphosis and becomes a lung-breather in the adult 



