ANIMALS AND THEIR ENVIRONMENTS. 317 



the fresh waters of Mexico, and despite its somewhat unin- 

 viting appearance, is used in its native regions for food. It 

 is a lizard-like animal ; possesses a fin-like flattened tail, 

 which forms an efficient swimming organ ; and as a further 

 adaptation to an aquatic existence, possesses three well- 

 developed and fringe-like gills on each side of its neck. 

 Lungs also exist in the axolotl, which is thus a most typical 

 amphibian, in so far as the possession of a double set of 

 respiratory organs is concerned. Its length is about ten or 

 twelve inches, and its colour a dark brown spotted with 

 black. The axolotl has been long known to science as an 

 interesting amphibian ; but the possibility that it was only an 

 immature or larval form of some other amphibian, formed 

 perhaps the most noteworthy point in its history. Cuvier 

 appears to have had doubts of its identity ; and Mr. Baird, 

 writing of the axolotl, thus says : " It so much resembles the 

 larva of Amblystoma punctata (a North American newt), in 

 both external form and internal structure, that I cannot but 

 believe it to be the larva of some gigantic species of the 

 genus." Nothing very definite, however, could be urged in 

 support of the idea that the axoltol was a creature still in 

 the days of its youth; and there existed, moreover, one 

 feature which strongly militated against such a supposition, 

 namely, that these animals were capable of perfectly repro- 

 ducing their species, since they were known to produce 

 young freely, both in a state of nature and captivity. Of all 

 physiological tests of an animal's maturity, this latter may be 

 said to be that of the most general application. The law 

 that the perpetuation of the species is a function of adult 

 life only, is, in fact, one of the most universal application. 

 But in 1857, Dumeril laid before the French Academy of 

 Sciences a communication in which he noted the instructive 

 fact that some thirty axolotls had mysteriously emigrated 

 from the water in which they lived peacefully with hundreds 

 of their neighbours, had shed their gills, cast off their skin, 

 and had assumed the colour and appearance of the genus 

 amblystoma (Fig. 64), a well-known group of American land- 



