r 1 4 URODELA CHAP. 



Then she takes a rest before proceeding again ; the whole process, 

 in which the male takes no further interest, lasting about two 

 days. The most suitable temperature is one of 18-20° C, or 

 about 68° F. The water must be well aerated. Sterile ei^us 

 turn white on the second day. The little larvae are hatched in 

 about a fortnight. Eggs which are kept in a higher tempera- 

 ture, from 22-24° C, develop more quickly, but the resulting 

 young are smaller ; they show already on the fifth day head, 

 tail, and the beginning of the gills. According to Bedriaga, 

 they live at first upon Infusoria and Daijlmia ; when they are 

 20-25 mm. long tliey eat Tulnfex rifulorum ; later on they take 

 scraped meat and are liable, when hungry, to nibble oft' each 

 other's gills, but these are easily reproduced. When 20-25 cm. 

 long, at the age of about six months, they are able to breed. 

 The chief point of interest is the fact that this species of AmUy- 

 stoma frequently remains throughout life in the larval state, except 

 that it develops generative organs. The natural causes of tliis 

 retention are not completely known. According to Sliufeldt, 

 who observed them under natural conditions near Fort Wingate 

 in Xew Mexico, plenty of food, the drying up of the swamps, and 

 the increasing temperature of the diminishing water, hurries on 

 the metamorphosis, while deeper water retards it. Weismann ^ 

 suggested that the specimens in the Mexican lakes whieli 

 remained Axolotls were prevented from becoming perfect 

 Amblystomas on account of these lakes, after tlie disappearance 

 of the surrounding forests, having receded from their former 

 boundaries, which are now covered with a saline, uninhal)ital)le 

 crust. This may be an explanation, altliough Axolotls do not 

 live in brackish water. But Weismann went farther, and with 

 his well-known dialectic powers has succeeded in spreading the 

 Ijelief not only that the Axolotl is a case of reversion to an 

 ancestral stage, but that the present Amhli/stoma, instead of 

 being the progressive, perfect form, is likewise a case of reversion. 

 A reversion from a reversion ! The whole line of evolution 

 would then be as follows: Amhlystoma ; its young, owing to 

 adverse circumstances, revert to the stage of the Perennibranchiate 

 ancestors of all Urodela ; if some of these Axolotls lose their gills 

 and fins, they revert thereby into tlie original Amhlystomn. 



' Zntschr. wiss. Zool. xxv. 1875, p. 297. See also Halm, Jiev. Quest. Sci. 

 (2), i. 1892, p. 178. 



