154 THE SHAPES AND SIZES OF ANIMALS 



accident that the Megatherium attained dimensions 

 very closely comparable to those of an elephant, 

 while the biggest fossil Ungulates were not far 

 short of this size. 



It has been suggested that the limits of size are 

 due to the nature of the materials of which animals 

 are constructed, and that the difficulty in increasing 

 that size is a mechanical one ; that just as it would 

 be impossible to construct a Forth Bridge of stone, 

 for in order to get sufficient strength it would be 

 necessary to employ so great a quantity of material 

 that the structure would be crushed by its own 

 weight, so the bones, ligaments, and muscles of 

 which the animal frame is constructed will permit 

 of size up to that of an elephant but not beyond. 

 It is not hard to find points in the anatomy of the 

 elephant that support this view. The several 

 joints of each limb, which in smaller quadrupeds are 

 in the natural standing position bent on one 

 another, often at considerable angles, are in the 

 elephant placed vertically one above another, an 

 arrangement that enables them to support with 

 less muscular effort the enormous weight of the 

 body and head. The massive pillar-like form of 

 the legs, which take up the greater part of the 

 space below the body, are also indications that the 

 limits of size are being approached, and when we 

 find that though approached closely time after time, 

 the size of the elephant has not been exceeded in 

 past times by terrestrial animals, save by a few 

 members of its own group, and with the possible 

 but as yet doubtful exception of Brontosaurus and 



