GREAT STEPS IN ORGANIC EVOLUTION 88i 



divergences above indicated are in some measure misleading, for 

 they contrast the modern ends of two sets of branches which have 

 been growing away from one another for millions of years. Secondly, 

 when we think of the ancient reptiles we must not picture a welter 

 of crocodiles and lizards, but a very motley assemblage, among which 

 in Mesozoic Ages there were all sorts of types and tentatives. Some 

 ran on the ground and others swam in the sea; some burrowed and 

 others climbed; some were bipeds, and others were almost snake- 

 like. There were heterogeneous types, testing all things, venturing 

 on all modes of life. 



Three lines of evidence converge towards the conclusion that 

 birds evolved from an ancient saurian stock — using this general 

 word so as to shift our thoughts away from our particular present- 

 day reptiles. There is evidence (a) from homologies of structure, 

 (b) from similarities of development, and (c) from annectant types. 



Deep resemblances in structure and development are termed 

 homologies, and we find that birds and reptiles agree in showing 

 many of these, thus: 



(i) a complex lower jaw, made up of 4-6 bones on each side, 

 whereas mammals have but one ; 



(2) an articulation of this lower jaw with the quadrate bone of 



the skull, whereas in mammals the articulation is with the 

 squamosal, and the quadrate is reduced to a small ear- 

 bone; 



(3) a single occipital condyle at the back of the skull, whereas 



amphibians and mammals have two ; 



(4) an inter- tarsal ankle-joint, whereas that of mammals is 



cruro-tarsal. 



This list of agreements might be carried much farther, but that 

 is not necessary for our present purpose. It is of interest, however, 

 to notice an occasional similarity in relatively unimportant details 

 — the straws which show how the evolutionary wind has blown. 

 Thus birds have in the front of their eyeball, at the junction of 

 cornea and sclerotic, a strengthening ring of bone, or of small bones, 

 the sclerotic ring; and this is also evident in many fossil reptiles, 

 such as the Fish-lizards or Ichthyosaurs. Many a bird before and 

 shortly after hatching has a small horny and calcareous "egg- 

 tooth", better called shell-breaker, a hardening at the tip of the 

 upper jaw, which is used in chipping the egg-shell, and is a good 

 instance of an effective structure used for only a few hours or 

 minutes and then discarded. It is of interest that this "shell-breaker" 

 occurs in some reptiles, such as the crocodile, and in that strange 

 living fossil, the New Zealand "lizard" (Sphenodon). It may be noted 

 in passing that this horny shell-breaker is not to be confused with 



VOL. II L 



