62 DOCTRINE OF EVOLUTION 



the affirmative, because evolution gives the only reason- 

 able explanation of such facts as these. The case 

 seems different from that of the frog, because gills 

 are used by the tadpole, but gill-slits and gill-bars can 

 have no conceivable value for the chick as organs 

 concerned with the purification of the blood. None 

 the less, if the transition from a gilled tadpole to the 

 adult with lungs means an evolution of amphibia from 

 fishlike ancestors, then the change of a chick embryo 

 with gill-clefts into the fledgling without them is most 

 reasonably interpreted as proof that birds as well as 

 amphibia have had ancestors as simple as fishes. 



As development progresses four small pads make 

 their appearance; two of these lie on either side of 

 the body back of the head and the other two arise near 

 the posterior end. They are far from being wings and 

 legs, but as day follows day they become molded into 

 somewhat similar limbs, as much alike in general plan as 

 the four legs of a lizard ; subsequently the ones at the 

 front change into real wings and the hinder ones be- 

 come legs. Meanwhile the internal organs slowly trans- 

 form from fishlike structures into things that display 

 the characteristics of reptilian counterparts, and only 

 later do they become truly avian. Last of all the finish- 

 ing touches are made, and the whole creature becomes a 

 particular kind of a bird which picks its way out of 

 the shell and shifts for itself as a chick. 



Only a few of the countless details have been men- 

 tioned which demonstrate the resemblance of the 

 successive stages first to fishes, and later to amphibia 

 and reptiles. We have a wide choice of materials, 

 but even the foregoing brief list of illustrations shows 



