228 



THE ORIGIN AND EVOLUTION OF LIFE 



Fig. 105. Skeleton of Archceoptcryx 

 (left) Compared with That of the 

 Pigeon (right). 



Showing the abbreviation of the tail into 

 the pygostyle and the conversion of 

 the grasping fore limb into the bones 

 of the wing. After Heilman. 



into long primary feathers on 

 the fore Umbs and at the sides 

 of the extended tail would afford 

 a glissant parachute support 

 for short flights from trees to 

 the ground (Fig. 106). Quite 

 recently a Jour-wing theory, the 

 tetrapteryx theory, has been pro- 

 posed by Beebe, based on the 

 observation of the presence of 

 great feathers on the thighs of 

 embryos of modern birds and 

 of supposed traces of similar feathers on the thighs of the old- 

 est known fossil bird, the Ar- 

 chceopteryx of Jurassic age. Ac- 

 cording to this hypothesis after 

 the four-wing stage was reached 

 the two hind-leg wings degen- 

 erated as the flight function 

 evolved in the spreading feathers 

 of the forearm-wings and the 

 rudder function was perfected ^^ , c ^ 7 >, 



^ Fig. 106. Silhouettes of Archaop- 



in the spreading feathers of the tcryx (A) and pheasant (B). 



tail (Fig. 107). Both of these ^^''^ °" '^' '^^^^^l '^'"'y- ^^'" 



Fig. 107. Four Evolutionary Stages in the Hypothetical Four-winged Bird. 



After Beebe. 



