Morphological J^ariation and Its Causes in A. tigrinum 9 



comprise four sets of factors, viz., diversities in nutrition, diver- 

 sities in habitual locomotion, diversity in the age at which meta- 

 morphosis occurs, and, in case of sexually mature larvae or or- 

 dinary adults, diversity of sex. I have repeatedly observed the 

 operation of each one of these factors singly ; and extreme types 

 of body form are always due, in my experience, to special com- 

 binations of appropriate factors from these classes. Thus the 

 most elongate forms of Amblystoma, speaking generally, are due 

 first to slow growth, second to the free-swimming habit, third to 

 the prolongation of larval life, and finally to the assumption of 

 sexual maturity as males, this last occurring in either the 

 branchiate (larval?) or the non-branchiate condition. 



The influence of a slow rate of growth is very marked indeed ; 

 although I ought, perhaps, to say an intermediate rate of growth ; 

 for if nutrition is checked to the point of approximate starvation 

 or to that wdiere the animal merely holds its own, the results are 

 again quite dififerent. I will not discuss these here. The rate of 

 growth to which I refer is, for an intermediate sized larva of 

 twelve to twenty centimeters, approximately a centimeter per 

 month. A rapidly growing summer specimen may more than 

 quadruple this amount. The easiest method of inducing this mod- 

 erate or slow growth is by winter feeding. Larvae which have 

 slipped through the late summer without metamorphosis pass into 

 a curious condition of low metabolism, or relative quiescence. It 

 is not hibernation by any means, but yet, as it were, an approach 

 to hibernation. The change is but partially, if at all, due to tem- 

 perature, and it is only very slowly and partially overcome by 

 temperature. Metamorphosis is almost impossible during the 

 early winter months, no matter to what conditions the larvae are 

 subjected. Such larvae, if kept at living-room temperatures, 

 readily take meat and thrive, although they will not eat more than 

 once or twice a week, whereas summer larvae under like condi- 

 tions require daily feeding to prevent immediate metamorphosis. 

 These slowly grown winter larvae invariably show a marked 

 change in form, the body, tail, and even the head increasing in 

 length, but hardly at all in width. The tail increases in length 

 more rapidly than the body. This change of form, although more 



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