20 /. H. Powers 



mer does not rob the former of its natural and complete inde- 

 pendence under appropriate conditions. 



Herewith I have spoken of the chief factors which control the 

 general bodily form, although I have dwelt chiefly upon the fac- 

 tors which produce elongated rather than shortened forms. The 

 production of the one is the reverse of the production of the 

 other. 



SPECIAL VARIATIONS IN THE FORM OF THE TAIL 



A glance at the tail contours on plates III and V, although the 

 forms figured illustrate by no means all of the phenomena, will 

 show that there is much to be explained. There are long tails 

 that are broad, and long tails that are narrow, and short tails 

 broad and narrow ; there are tails that are thick and fleshy, and 

 tails that are thin; there are (at least in animals out of my ex- 

 periments) tails almost round and tapering to short, sharp tips. 

 And with all these there is a host of minor though not unimpor- 

 tant variations. I shall discuss only a part of them. 



Length having been already explained, I will next speak of the 

 matter of breadth. Fairly extreme' cases of varying tail breadth 

 are shown on plate III, figures i and 2. The explanation of such 

 differences offered by Baird and approved by Cope is that they 

 are caused by the varying degrees of aquatic habit of the adults. 

 Thus Baird is quoted by Cope as saying: "A more persistent 

 residence in water is shown by the broader and more depressed 

 digits, higher and more compressed tail, and more or less decided 

 ridge (sometimes even membranous). I have no doubt that an 

 animal while possessing these features in marked degree when in 

 water would lose them to a measurable extent after a lengthened 

 residence on land. This aquatic habit is generally greatest dur- 

 ing the breeding season." I am glad to be able to confirm in 

 some degree these assumptions, and I have been to the trouble of 

 keeping several lots of adult amblystomas for fully two vears in 

 water of different depths, besides feeding a great many other 

 adults, both young and old, in water for varying lengths of time 

 up to four months. I have also taken pains to ascertain as care- 



2X6 



