III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 81 



Either, birds must have had two distinct origins whence 

 they grew to their present conformity, or the very same 

 skeletal, and probably cerebral, characters must have 

 spontaneously and independently arisen. Here is a 

 dilemma, either horn of which bears a threatening as- 

 pect to the exclusive supporter of "Natural Selection," 

 and between which it seems somewhat difficult to 

 choose. 



It has been suggested to the author that this difficulty may 

 be evaded by considering pterodactyles and carinate birds as 

 independent branches from one side of an ancient common 

 trunk ; while similarly the Dinosauria and struthious birds 

 are taken to be independent branches from the other side 

 of the same common trunk ; the two kinds of birds resem- 

 bling each other so much on account of their later develop- 

 ment from that trunk as compared with the development 

 of the reptilian forms. But to this it may be replied, that 

 the ancient common stock could not have had at one and 

 the same time a shoulder structure of both kinds. It must 

 have been that of the struthious birds or that of the 

 carinate birds, or something different from both. If it 

 was that of the struthious birds, how did the pterodactyles 

 and carinate birds independently arrive at the very same 

 divergent structure ? If it was that of the carinate birds, 

 how did the struthious birds and Dinosauria independently 

 agree to differ ? Finally, if it was something different from 

 either, how did the carinate birds and pterodactyles take 

 on independently one special common structure when dis- 

 agreeing in so many ; while the struthious birds, agreeing 

 in many points with the Dinosauria, agree yet more with 

 the carinate birds ? Indeed, by no arrangement of branches 

 from a stem can the difficulty be evaded. 



