lO THE AMERICAN AA I'L RALlsr. [Xoi.. XXW 1 1 1. 



The tail is generally reduced as a result of cursorial adaptation, 

 though in coursing dogs, as the grayhound and pointer, it aids in 

 keeping the balance when the creature changes its direction and 

 this may be a secondary cause for its retention. In saltatorial 

 forms on the contrary, the tail becomes an important organ foi- 

 use as a counterpoise ; for in truly saltatorial mammals the tail 

 increases in length and in weight directly with the increase in 

 proportion of the hind over the fore limbs. It is in such bipedal 

 forms as the kangaroos and the jerboas that the caudal counter- 

 poise reaches its highest expression, for in the former the tail is 

 long and heavy while in the latter the somewhat less proportion- 

 ate weight is compensated for by the extreme length of the 

 organ and the tuft of hair at its tip. The tail of the African 

 jumping hare, Pedetes, is long and feather-like, like that of a 

 squirrel. The development of a caudal counterpoise in bipedal 

 mammals is paralleled among dinosaurs of the Mesozoic though, 

 as the author will show in a forthcoming memoir,^ dinosaurs are 

 never saltatorial, but always progress by alternating strides. 

 This is what one would be led to infer, for whatever the increase 

 in speed may be, I know of no reptile which runs at a gallop 

 that is with each pair of limbs moving nearly in unison, while 

 among the mammalia this is the common method of rapid pro- 

 gression. The jerboas zvalk on the hind limbs with alternate 

 strides, hopping only when speeding while the kangaroos have 

 lost the more primitive alternate footed gait and use the hop for 

 all degrees of rapidity. The hop may thus be considered as a 

 sort of bipedal gallop. 



Among the struthious birds, the cursorial habit evidently 

 having been acquired after the loss of the reptilian tail, the 

 counterpoising function is subserved by the wings which bear 

 up the anterior part of the body and at the same time lighten 

 the creature's weight. 



