CONCLUSION. 189 



The Birds would appear to have branched off by a single stem, which 

 gradually lost its reptilian characters as it assumed the ornithic type, and 

 in the existing Ratitse we have the survivors of this direct line. The lineal 

 descendants of this primal stock doubtless early attained feathers and warm 

 blood, but, as already shown (p. 114), never acquired the power of flight. 

 The volant birds doubtless separated early from the main avian stem, 

 probably in the Triassic, since, in the formation above, we have 

 ArchcBojjteri/x, with imperfect powers of flight. 



This power of flight probably originated among the small arboreal 

 forms of reptilian birds. How this may have commenced, we have an 

 indication in the flight of Galeopithecus, the flying squirrels (Pteromys), 

 the flying lizard (Draco), and in the flying tree-frog (Rliacoplwrus). In the 

 early arboreal birds, winch jumped from branch to branch, even rudi- 

 mentary feathers on the fore limbs would be an advantage, as they would 

 tend to lengthen a downward leap, or break the force of a fall. As the 

 feathers increased, the body would become warmer, and the blood more 

 active. With still more feathers, would come increased power of flight, 

 as we see in young birds of to-day. A greater activity would result in 

 a more perfect circulation. A true bird would doubtless require warm 

 blood, but would not necessarily be hot-blooded, like the birds now living. 



The short wings and clumsy tail of Archceopteryx were quite sufficient 

 for short flights from tree to tree, and if the body were essentially naked, 

 as now supposed, we have in this Jurassic form an interesting stage in 

 the development of birds before full plumage was attained. Whether 

 Archceopteryx was on the true Carinate line cannot at present be determined, 

 and this is also true of Ichthyornis ; but the biconcave vertebrae of the latter 

 evidently suggest that this form was an early offshoot. It is probable that 

 Eesperornis came off from the main Struthious stem, and has left no 

 descendants. 



These three ancient birds, so widely different from each other, and from 

 all modern birds, prove beyond question the marvelous diversity of the 

 avian type in Mesozoic time ; and also give promise of a rich reward to 

 the explorer who successfully works out the life-history of allied forms, 

 recorded in ages more remote. 



