DISTRIBUTION OF BIRDS IN TIME AND SPACE. 185 



different from that of the rooks and crows of to- 

 day. The fashion of wing-bones is the same 

 as theirs. Where it differs from them, its 

 counterpart can be found in living birds. 



These are points of great significance. We 

 have no reason to suppose that they reached 

 this particular form by any other than the 

 ordinary routes, which were then as now. The 

 wing and foot as we see them in the fossil must 

 be the result of a gradual modification of some 

 other pre-existing type of fore- and hind-limb. 



This modification must have been a very very 

 slow one, and pre-supposes a long line of ancestors 

 stretching back far beyond the Jurassic period. 



The argument for descent with modification, 

 which is the outcome of the evolution hypothesis, 

 may be admitted to have received strong support 

 then from this skeleton. 



The two birds now to be discussed afford us 

 additional evidence, for in the one we see what 

 evolution has accomplished by specialisation 

 accompanied by degeneration; in the other 

 we have an object lesson in evolution with 

 progression, and in both we have evidence of 

 a distinct advance in type. Both the forms 

 moreover are contemporaries, showing that the 

 two were being evolved at the same time. 



These are Ichthyornis and Hesperornis of the 

 cretaceous period. 



Both of these were more advanced in type 

 than Archceopteryx. In Ichthyornis the form of 

 the tail and wing is like that of modern birds, 

 so is the pelvis. Its powers of flight were 

 probably vastly superior, for its breast-bone was 



