VARIATION AND ADAPTATION 217 



incapable of being affected by conditions in one direction or the 

 other. 



Kollmann has shown that neoteny occurs in a large 

 number of species and he distinguished between cases in which 

 the metamorphosis is only temporarily postponed, the tadpoles 

 passing through the winter in the larval condition and passing 

 through their metamorphosis in the following summer, and 

 cases in which the animal becomes sexually mature in the gill- 

 breathing condition. It is a curious fact that examples of the 

 first kind occur in Anura and of the latter only in Urodela. The 

 following are the species in which neoteny has hitherto been 

 observed : I. Partial Neoteny. Pelobates fuscus, Bombinator 

 pachypus, Pelodytes punctatus, Alytes obstetricans, Hyla arborea, 

 Rana esculenta, Rana temporaria, Bnfo vulgaris and B. viridis. 

 II. Total Neoteny. Triton vulgaris, T. alpestris, T. crista- 

 tus, T. boscai, T. waltlii, and Ambly stoma tigrinum. 



Dr. Hans Gadow of Cambridge University has recently 

 studied the axolotl in its native habitat and in his charming 

 book Through Southern Mexico, gives a most interesting de- 

 scription from his own observations of the conditions under 

 which it lives at the present time. About four miles to the 

 east of the city of Mexico is the Lake of Texcoco which at 

 the time of the conquest by Cortes extended to the city 

 itself. The original native city was intersected by canals, was 

 in fact a city of lake-dwellings, and as the plain in which the 

 city lies is surrounded by mountains with numerous streams 

 running into the lakes the latter were liable to spread in exten- 

 sive inundations. At present great drainage works keep the 

 level of the water fairly constant and the city is situated on 

 dry land. The water of Texcoco is brackish and during the 

 dry season the land from which it retreats is covered with a 

 saline crust. The lake is " a dreary waste of water only enli- 

 vened in the autumn by numerous waterfowl and it contains 

 several kinds of small fish, but there are no axolotls in it, the 

 saltness of the water making it quite uninhabitable to any 

 Amphibia. This fact effectually disposes of the suggestion 

 made by Weismann that the axolotls were compelled to 

 remain in the water because the surrounding shores were 

 covered with a saline crust which would be fatal to the terres- 

 trial form ; there could scarcely be a saline deposit on the 



