III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 85 



of tlic Diiiosauria with striiihious birds; (3) that of the 

 cariiiatc and struthious birds with each other. 



Either birds must have had two distinct origins whence 

 they grew to their present conformity, or tlie very same 

 skeletal, and probably cerebral characters, must have spon- 

 taneously and independently arisen. Here is a dilemma, 

 either horn of which bcnrs a threatening as})ect to the 

 exclusive supporter of " Natural Selection," and between 

 which it seems somewhat difficult to choose. 



It has been suggested to me that this difficulty may be 

 evaded by considering pterodactyls and carinatc birds as 

 independent branches from one side of an ancient common 

 trunk, while similarly the Dinosauria and struthious birds 

 arc tnken 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 kmds. It must 

 have been that of the struthious birds or that of the c<^ri- 

 nate birds, or something different from both. If it was that 

 of the struthious birds, how did tlie pterodactyls and cari- 

 nate 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 cither, 

 how did the carinate birds and pterodactyls take on inde- 

 pendently one special common structure when disagreeing 

 in so many ; while the struthious birds, agreeing in many 

 points with the Dinosauria, agree yet more with the cari- 

 nate birds ? Indeed, l)y no arrangement of branches from a 

 stem can the difficulty be evaded. 



Prof. Huxley seems incHned ' to cut the Gordian knot 



' "Trocccdings of Geological Society," November, 18G9, p. 38. 



