No. 406.] DINOSAUR-AVIAN STEM. 795 
ancestral to all Archosauria, including birds, the crucial ques- 
tion remains, whether birds sprang off independently from a 
proganosaur stem or from a common dinosaur-avian stem. In 
the origin of the birds we have to imagine, first, a terrestrial 
stage, in which bipedal was gradually substituted for quadru- 
pedal progression ; it would appear probable that the bipedal 
progression was first acquired during a terrestrial stage, because 
the foot of birds is primarily a walking, and not a climbing, 
organ ; second, a cursorial bipedal or, more probably, an arbo- 
real stage, in which both fore limb and tail enjoyed a change of 
function contemporaneous with the acquisition of feathers. 
VII. CORRELATED DEVELOPMENT OF TRIDACTYLISM AND 
BIPEDAL PROGRESSION. 
It appears probable that the ancestral dinosaur was a quad- 
rupedal type, with the body well raised off the ground, distinc- 
tively a land animal, because the distinctive specialization of 
this group appears to have been terrestrial, the Cetiosauria 
or Sauropoda secondarily acquiring an amphibious mode of 
ife. The manner in which the four-footed primitive dinosaurs 
acquired the bipedal habit and consequent reduction of the fore 
limbs and elongation of the hind limbs is beautifully illustrated 
in Chlamydosaurus of Australia and some other living lizards. 
As observed by Saville-Kent, this animal in all its rapid move- 
ments raises the fore limbs, balances the anterior part of the 
body with the tail, and runs along rapidly upon the hind limbs. 
This analogy appears to demonstrate that an important func- 
tion of the tail was to serve as a balancing organ. (We note 
in parenthesis that this function is developed among birds.) 
Kent remarks further: * Such is the construction of the hind 
foot and its component digits that, when thus running, the 
central digits only, rest upon the ground. As a consequence 
of this structural peculiarity, the track made by this lizard 
When passing erect over damp sand or other impressible soil 
would be tridactyl like that of a bird, and would also corre- 
spond with the tracks that are left in Mesozoic strata by vari- 
Ous typical Dinosauria. This tridigitigrade formula of the 
