THE ANCESTRY OF BIRDS. 61 1 



teeth, tbe like of which of course exist in no living bird. The skele- 

 ton is for the most part reptilian ; and, though the legs are bird-like, 

 they are not much more so than those of compsognathus, an unmixed 

 reptile. Even the wings are more like the fore-legs, and could only 

 be used for flight by the aid of a side membrane. Accordingly, we 

 may say that we have lithographed for us in archaeopteryx a specimen 

 of the intermediate state, when reptiles were just in the very act of 

 passing into birds. The scales and protuberances on the body had 

 already developed into feathers ; the fore-legs had already developed 

 into rude and imperfect wings, and the feet had become decidedly 

 bird-like ; but as yet there was only a very small breast-bone, the tail 

 remained in internal structure like that of a lizard, the jaws still con- 

 tained pointed teeth, and the wing ended in a three-toed hand, while 

 flight was probably as rudimentary as in the flying-lemur and the fly- 

 ing-squirrel. Nowhere in the organic series has geology supplied us 

 with a better missing link than this uncouth and half-formed creature, 

 Nature's first tentative rough draft of the beautiful and exquisitely 

 adapted modern birds. 



Such an animal, once introduced, was sure to undergo further modi- 

 fication, to fit it more perfectly for its new sphere of action. In the 

 first place, the tail was sure to grow shorter and shorter, by stress of 

 natural selection, because a more fan-like organ would act better as a 

 rudder to steer the flight than the long lizard-like tail of archseopteryx. 

 In the second place, the general bony structure was sure to grow bet- 

 ter adapted for flight, by the development of some such feature as the 

 keeled breast-bone, and the general modification of the other parts 

 (especially the wing) into better correspondence with their new func- 

 tion. At the same time, it must not be supposed that all intermediate 

 birds would lose their reptilian features equally and symmetrically. 

 Some for a time might retain one lizard-like peculiarity, say the teeth, 

 and some might retain another, say sundry anatomical points in the 

 structure of the skeleton. It was long indeed before the whole tribe 

 of birds acquired the entire set of traits which we now regard as char- 

 acteristic of their class. During the intervening period they kept 

 varying in all directions, tentatively, if one may say so, and thus the 

 early forms of birds differ far more among themselves than do any 

 modern members of the feathered kingdom. In other words, when 

 the full bird type was finally evolved, it proved so much better adapted 

 to its airy mode of life than any other and earlier creature that it lived 

 down not only the rude reptilian pterodactyls but also the simpler 

 primeval forms of birds themselves : exactly as civilized European 

 man is now living down not only the elephants and buffaloes but the 

 red Indian and the Australian black fellow as well. 



Some of the varying primeval forms have been preserved for us as 

 fossils in the chalk deposits of the Western States, which are of course 

 later in date than the oolitic slates of Solenhofen, where we find the 



