708 ON FORM AND MECHANICAL EFFICIENCY [ch. 



the anterior arm. The great cantilever girder appears, accordingly, 

 balanced over the hind-legs. It is now constituted in part by 

 the posterior dorsal or lumbar vertebrae, all traces of special 

 elevation having disappeared from the anterior dorsals; but the 

 greater part of the girder is made up of the great iliac bones, 

 placed side by side, and gripping firmly the sacral vertebrae, often 

 almost to the extinction of these latter. In the form of these 

 iliac bones, the arched curvature of their upper border, in their 

 elongation fore-and-aft to overhang both ways their supporting 

 pier, and in the coincidence of their greatest height with the 

 median line of support over the centre of gravity, we recognise 

 all the characteristic properties of the typical balanced canti- 

 lever*. 



(7) We find a highly important corollary in the case of 

 aquatic animals. For here the effect of gravity is neutralised ; 

 we have neither piers nor cantilevers; and we find accordingly 

 in all aquatic mammals of whatsoever group — whales, seals or 

 sea-cows— that the high arched vertebral spines over the withers, 

 or corresponding structures over the hind-limbs, have both 

 entirely disappeared. 



Just as the cantilever girder tended to become obsolete in the 

 aquatic mammal so does it tend to weaken and disappear in the 

 aquatic bird. There is a very marked contrast between the high- 

 arched strongly-built pelvis in the ostrich or the hen, and the 

 long, thin, comparatively straight and weakly bone which repre- 

 sents it in a diver, a grebe or a penguin. 



But in the aquatic mammal, such as a whale or a dolphin (and 

 not less so in the aquatic bird), stiffness must be ensured in order 

 to enable the muscles to act against the resistance of the water 

 in the act of swimming; and accordingly nature must provide 

 against bending-moments irrespective of gravity. In the dolphin, 

 at any rate as regards its tail end, the conditions will be not very 

 different from those of a column or beam with fixed ends, in 

 which, under deflexion, there will be two points of contrary 

 flexure, as at C, D, in Fig. 351. 



* The form of the cantilever is much less typical in the small flying birds, 

 where the strength of the pelvic region is insured in another way, with which we 

 need not here stop to deal. 



