62 COSMIC FMILOSi iPHY, [pr. ii. 



a much stronger case has been made out in favour of the 

 descent of the struthious birds from large reptilian forms 

 akin to the diuosauria, — of which extinct order the member 

 most commonly known is the gigantic iguauodon. Kow 

 here, says Mr. Mivart, is a dilemma just like the one which 

 confronted us in the case of mammals. If all birds started 

 from the pterodactyl, why do the struthious birds so strongly 

 resemble a totally different reptile ? If all birds started from 

 a dinosaurus, why do tlie carinate birds so strongly resemble 

 the pterodactyl ? If we try to split the difference, and say 

 that the carinate birds started from the pterodactyl, while 

 the struthious birds started from the dinosaurus, the difficulty 

 is immensely increased. For then the question arises, how 

 could the struthious and the carinate birds, starting from 

 suoh different points, have come to resemble each other 

 so strongly ? 



Mr. Mivart is careful to state that these zoological cross- 

 relations do not constitute an obstacle to the theory of evolu- 

 tion. They are difticulties only on the theory that organic 

 evolution has been solely caused by the natural selection of 

 fortuitous variations. To make this more clear, let us pro- 

 visionally accept one of each of the pairs of alternatives 

 offered by the two cases just described. Let us agree, with 

 Prof Haeckel, that all the monodelphian mammals have 

 come from one didelphian; and let us agree, with Prof. 

 Huxley, that the kinship between birds and reptiles is closest 

 in the case of the struthious birds and the dinosaurians. 

 Now we are obliged to maintain that the original monodel- 

 phian branched off into a dozen or more forms, of which six 

 or seven happen to agree remarkably, in general appearance 

 and in habits of life, with six or seven of the forms into 

 which the original didelphian had at an earlier date branched 

 off. And we are also obliged to maintain that the remark- 

 able shoulder-structure of the pterodactyl, in which it agrees 

 so closely with the carinate birds, was independently evolved 



