THE ORIGIN OF VERTEBRATED ANIMALS i8r 



evidence given by development, indications of both 

 processes being present in the development of the 

 pelvic girdle in birds. Again, fossil birds, such as 

 the Archceopteryx, are known which possess teeth 

 and biconcave vertebrae, a long and many-jointed 

 tail, and free metacarpal bones ; thus showing the 

 convergence between birds and reptiles as we pro- 

 ceed backwards in time. 



The actual origin of wings is still doubtful. The 

 fore-limbs seem first to have become small, as is the 

 case among the Dinosaurs, and then in some 

 unknown way to have changed their function and 

 become wings. Ostriches are said sometimes to 

 give a possible clue, but more probably they are 

 forms in which the power of flight has been lost, for 

 wings are not essential to the bird type. The 

 anatomy of birds supports the contention, for in 

 most reptiles, although both left and right aortic 

 arches persist, yet the right is much the larger. 

 The presence of one condyle to the skull, the 

 quadrate bone, and the absence of vertebral epi- 

 physes are other points in favour of this view. 



Evidence is also afforded by embryology. Birds 

 jpass through the gill-cleft stage, but have no gills ; 

 therefore it is probably either to mammals or reptiles 

 that we must look for allies. The large eggs of 

 birds, the presence of egg-shells, and the details of 

 their early development point to reptiles, as also 

 does the development of the optic lobes of the brain, 

 which though laterally placed in the adult bird, are 

 dorsal in the embryo, and exactly like those of the 

 lizard. The development of the metacarpal bones 



