VARIATION AND ADAPTATION 219 



condition, and yet like the axolotl it never leaves the water but 

 is aquatic throughout its life, both in the gill-breathing and the 

 lung-breathing condition. Nothing could prove more con- 

 clusively that the neoteny of the Axolotl is not due merely to 

 the fact that it remains in the water, and it cannot be suggested 

 that Ambly stoma altamirani metamorphoses because it has less 

 food than the Axolotl, for it finds enough food in the water, in 

 its adult air-breathing state. Why then of these two species, 

 both permanently aquatic, does one remain a gill-breather and 

 the other change into a lung-breather ? The question cannot be 

 answered completely until the physical conditions under which 

 they live respectively have been thoroughly analysed and com- 

 pared ; but there are indications in Dr. Gadow's description 

 that the real explanation lies in the difference between the 

 waters in the two cases with regard to the dissolved oxygen. 

 Lake Xochimilco contains besides living plants a great deal of 

 decomposing vegetable matter and this would naturally remove 

 much oxygen from the water and give off carbon dioxide in 

 return ; but evidently since the water contains numbers of fish 

 and larval insects it must be highly charged with oxygen. The 

 lake receives few streams, but numerous springs of the clearest 

 water rise up in various places from the bottom especially at 

 the southern end where they are very deep ; it is probable that 

 these springs, whatever their origin, are saturated with oxygen 

 and the lake would thus receive a constant supply of this ele- 

 ment at the bottom, whereas in other cases, when oxygen is 

 only obtained from the surface, decomposing vegetation in a 

 warm climate removes all the oxygen from the deeper layers 

 of the water. On the other hand, it is not so easy to find evi- 

 dence that the water in which the larvae of the metamorphosing 

 species, Amblystovia altamirani, live, is deficient in oxygen. 

 Dr. Gadow states that both larvae and adults were found in 

 June and September in the cool rushing streams, and such 

 streams are not likely to be deficient in oxygen. Altitude might 

 make some difference, these streams being about 8000 feet above 

 the sea while the city of Mexico is about 7400 feet ; not a very 

 great difference. Dr. Gadow, however, remarks that the larvae 

 are not, like the adults, restricted to the clear streams, but lived 

 also in quiet water which was muddy and overgrown with 

 watercresses and similar plants ; such water might very probably 



