1006 ON FORM AND MECHANICAL EFFICIENCY [ch. 



this process going so far as to relieve its fore-limbs of all weight 

 whatsoever. This would seem to be the case in such a form as 

 Diplodocus, ■ and also, in Stegosaurus, whose restoration by Marsh 

 is doubtless substantially correct*. The fore-limbs, though com- 

 paratively small, are obviously fashioned for support, but the weight 

 which they have to tarry is far less than that which the hind-limbs 

 bear. The head is small and the neck short, while on the other hand 

 the hind-quarters and the tail are big and massive. The backbone 

 bends into a great double-armed cantilever, culminating over the 

 pelvis and the hind-limbs, and here furnished with its highest and 

 strongest spines to separate the tension-member from the com- 



Fig. 481. Diagram of Stegosaurus. * 



pression-member of the girder. The fore-legs form a secondary 

 supporting pier to this great continuous cantilever, the greater part 

 of whose weight is poised upon the hind-limbs alone. 



(6) In the slender body of a weasel, neither head nor tail is such 

 as to form an efficient cantilever; and though the lithe body is 

 arched in active exercise, our parallel of the bridge no longer works 

 well. What else to compare it with is far from clear; but the 

 mechanism has some resemblance (perhaps) to an elastic spring. 

 Animals of this habit of body are all small; their bodily weight 

 is a light burden, and gravity becomes an ineffectual force, 



* This pose of Diplodocus, and of other Sauropodous reptiles, has been much 

 discussed. Cf. {int. al.) 0. Abel, Ahh. k. k. zool. hot. Ges. Wien, v, 1909-10 (60 pp.); 

 Tornier, SB. Ges. Naturf. Fr. Berlin, 1909, pp. 193-209; 0. P. Hay, Amer. Nat. 

 Oct. 1908; Tr. Wash. Acad. Sci. xlii, pp. 1-25, 1910; Holland, Amer. Nat. May 

 1910, pp. 259-283; Matthew, ibid. pp. 547-560; C. W. Gilmore [Restoration of 

 Stegosaurus), Pr. U.S. Nat. Miiseum, 1915. 



