University Studies 



Vol. VII JULY igoj No. 3 



I. — Morphological Variation and Its Causes in Amblystoma 



Tigrirmw} 



BY J. H. POWERS 



lit the American Naturalist for June, 1903, the writer pubUshed 

 an article which dealt with the causes of variation in the time of 

 metamorphosis in Amblystoma tigrinum. During the observations 

 and experiments which led to this article, and still more during 

 a long series of experiments subsequent to its publication, occasion 

 was offered to note the great variation in this species in its specific 

 and even generic characters. A record of these observations was 

 begun, as well as a collection of specimens, which latter has been 

 supplemented during the last two seasons by an extended series 

 of photographs. 



To record these variations is an easy matter, but at the same 

 time, in the writer's opinion, a matter of secondary importance. 

 The extreme variability of the species has long been known and 

 has been fairly well recorded, though still inadequately, by Baird 

 and Cope. To ascertain the nature and the causes of these varia- 

 tions, however, is at once a more important and a more difficult 

 task. 



Baird and Cope have, it is true, themselves pronounced very 

 decided opinions upon the causes of many of the variations in 



^Studies from the Zoological Laboratory, The University of Nebraska, 

 under the direction of Henry B. Ward, No. 71. 



197 



