XVI] IN THE SKELETON OF QUADRUPEDS 1005 



compressional stress transmitted almost directly downwards through 

 the neck: in correlation with which we observe that the bodies of 

 the cervical vertebrae are exceptionally large and strong, and steadily 

 increase in size and strength from the head downwards. 



(4) In the kangaroo, the fore-limbs are entirely relieved of theii* 

 load, and accordingly the tall spines over the withers, which were so 

 conspicuous in all heavy-headed quadrupeds, have now completely 

 vanished. The creature has become bipedal, and body and tail 

 form the extremities of a single balanced cantilever, whose maximal 

 bending-moments are marked by strong, high lumbar and sacral 

 vertebrae, and by iliac bones of peculiar form, of exceptional strength 

 and nearly upright position. 



Precisely the same condition is illustrated in the Iguanodon, and 

 better still by reason of the great bulk of the creature and of the 

 heavy load which falls to be supported by the great cantilever and 

 by the hind-legs which form its piers. The long heavy body and 

 neck require a balance-weight (as in the kangaroo) in the form* of 

 a long heavy tail; and the double-armed cantilever, so constituted, 

 shews a beautiful parabolic curvature in the graded heights of the 

 whole series of vertebral spines, which rise to a maximum over the 

 haunches and die away slowly towards the neck and towards the 

 tip of the tail. 



(5) In the case of some of the great American fossil reptiles such 

 as Diplodocus, it has always been a more or less disputed question 

 whether or not they assumed, like Iguanodon, an erect, bipedal 

 attitude. In all of them we see an elongated pelvis, and, in still 

 more marked degree, we see elevated spinous processes of the 

 vertebrae over the hind-limbs ; in all of them we have a long heavy 

 tail, and in most of them we have a marked reduction in size and 

 weight both of the fore-limb and of the head itself. The great size 

 of these animals is not of itself a proof against the erect attitude; 

 because it might well have been accompanied by an aquatic or 

 partially submerged habitat, and the crushing stress of the creature's 

 huge bulk proportionately relieved. But we must consider each 

 such case in the whole light of its own evidence ; and it is easy to 

 see that, just as the quadrupedal mammal may carry the greater 

 part but not all of its w^eight upon its fore-limbs, so a heavy-tailed 

 reptile may carry the greater part upon its hind-limbs, without 



