VARIATION AND ADAPTATION 213 



limbs are well developed but rather slender, with five toes on the 

 hind-foot, four on the fore-foot. The head is depressed with a 

 somewhat pointed snout ; the tail is compressed from side to 

 side and has a broad fin-membrane which extends dorsally 

 along the body almost to the neck. These axolotls reach a 

 length of eight or nine inches or even sometimes as much as a 

 foot ; they are never known in the lakes near Mexico to go 

 through any metamorphosis to a terrestrial condition, but pass 

 their whole lives in the water and reproduce in the water-breath- 

 ing condition. Originally therefore they were regarded as 

 " Perennibranchiata " and under the name Siredon pisciforme 

 were placed in the same division as Proteus and Necturus. In 

 the year 1865 some axolotls which had been kept for a year in 

 the Jardin des Plantes at Paris began to breed, the eggs which 

 they laid hatched, and in six months developed into full-grown 

 axolotls. This was complete confirmation of the belief that the 

 axolotl was sexually mature in the gill-breathing condition, but 

 this was not all : several of the young axolotls thus reared went 

 through a metamorphosis, the gills disappeared, the clefts closed 

 up, the fin-membrane disappeared, the head became broader and 

 the animals left the water and became entirely terrestrial. It was 

 thus shown that the axolotl was capable of changing into an air- 

 breathing form which would naturally belong to the family 

 Salamandridag. It was therefore a probable conclusion that 

 this terrestrial form was the species from which the axolotl was 

 descended and that the latter was a larval form which for some 

 reason had become permanent and sexually mature, but still re- 

 tained the power of undergoing metamorphosis under suitable 

 conditions. The most remarkable fact, however, was that the 

 gill-less creature into which the axolotls were converted were 

 not new, and unknown, but simply specimens of a well-known 

 Salamandrine species which is common all over the United 

 States from New York to California, a species named Ambly- 

 stoma tigrimnn. The Mexican axolotl is therefore simply the 

 persistent and sexually mature larva of A. tigrinum ; in other 

 parts of North America the larva undergoes its metamor- 

 phosis while still small, and the animal does not become sexu- 

 ally mature until after this metamorphosis ; in the Mexican 

 lakes the larva continues to grow and breeds without passing 

 through any metamorphosis at all. It is a curious fact that 



