Morphological Variation and Its Causes in A. tigrinum 15 



as much as do distinct species. Our discussion of the results of 

 cannibahsm will enforce this. 



Regarding the third factor which I have mentioned as a cause 

 for variation in general body proportions, viz., the age at which 

 the animal metamorphoses, little need be said. The facts are 

 simple. Indeed, considering the enormous variation in the size 

 at which A. tigrinum may undergo metamorphosis, it is surprising 

 that the resulting adults do not vary much more from this cause 

 than proves to be the case. I have had hundreds of larvae, in 

 ponds as well as in aquaria, which metamorphosed at ten centi- 

 meters in length, or even considerably less. The resulting adults, 

 sometimes weighing but three or four grams, are tiny things com- 

 pared to the giant larvae which may grow in one summer to 

 weigh 180 grams. My largest larva, three years old, has now 

 broken all records with a length of somethirfg more than 33 cm. 

 This larva will probably never metamorphose; but other giants, 

 which exceeded the size hitherto recorded for adults, after breed- 

 ing at the customary time in the spring, promptly defied the tra- 

 dition as to the perfect axolotl, by metamorphosing; and this 

 in spite of the cold running water and the darkness in which they 

 were kept. As already stated, these great variations in metamor- 

 phosis are of rather minor morphological consequence. Indeed, 

 in certain organs, noticeably the palatal teeth, I find that the ex- 

 tremes of precocity and of delay in metamorphosis produce well- 

 nigh invariably the same type, and a type not frequently found 

 among, the intermediate conditions. In general body proportions, 

 however, this is not the case, the small adult resulting from early 

 metamorphosis being larger headed and shorter tailed than the 

 larger adult resulting from later metamorphosis. Even this dif- 

 ference, however, so far as it relates to the tail, is little more than 

 a special case under the general law of slow and rapid growth. 

 The small larva (if it is capable of metamorphosis) has grown 

 rapidly; the larger larva has grown more slowly, at least during 

 the later portion of its development. 



The fourth factor which I have indicated as influencing the 

 general form is the factor of sex and sexual maturity. As I have 

 said, there is probably a certain average sex dimorphism even 



211 



