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THE AMERICAN NATURALIST [Vol. XLIV 



A second point that does not seem to have been brought 

 out in the discussion is that the dinosaurs are distin- 

 guished from other reptiles by the relatively large size 

 of the legs as compared with the body. In this respect 

 they have the proportions of mammals and birds. The 

 significance of this would seem obvious. A crocodile, 

 turtle or lizard crawls habitually, because his legs are not 

 large enough to carry the weight of his body. In dino- 

 saurs, as in mammals and birds, the legs are large enough 

 to carry the body comfortably clear of the ground and 

 presumably served that purpose. 



The nearest approach to the dinosaur proportions is 

 seen in some of the lizards, and it is just among these 

 that we find a tendency to lift the body from the ground, 

 especially during running. 



Another general consideration lies in the question of 

 the primary adaptation of the dinosaurs. Dr. Hay very 

 justly remarks that it is by no means necessary to sup- 

 pose that the bird-like (bipedal) dinosaurs had passed 

 through a mammal-like (quadrupedal) stage of evolution. 

 Indeed, if we regard the lizard as illustrating early stages 

 of a similar adaptation, the evidence would be just the 

 other way; the bipedal stage in dinosaurs came first, the 

 quadrupedal stage was a secondary adaptation. This is 

 generally admitted as regards the quadrupedal dinosaurs 

 of the Predentate group ; the reviewer believes that it is 

 also true of the Sauropoda, although the indications of 

 former bipedalism are less apparent in this group. The 

 hypothesis would serve, however, to explain several odd 

 features in their construction — e. g., the combination of 

 everted elbow with straight knee— and would connect 

 them more definitely with the Theropoda of the Triassic, 

 to which they are structurally traceable, as von Huene 

 has demonstrated. 



