III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 95 



therefore, complex and similar organs of diverse and inde- 

 pendent origin. 



BIKD'S-HEAD PROCESSES VERY GEEATLY ENLARGED. 



In the highest class of animals (the Mammalia) we have 

 almost always a placental mode of reproduction, i. e., the 

 blood of the foetus is placed in nutritive relation with the 

 blood of the mother by means of vascular prominences. 

 No trace of such a structure exists in any bird or in any 

 reptile, and yet it crops out again in certain sharks. There 

 indeed it might well be supposed to end, but, marvellous 

 as it seems, it reappears in very lowly creatures ; namely, 

 in certain of the ascidians, sometimes called tunicaries or 

 sea-squirts. 



Now, if we were to concede that the ascidians were the 

 common ancestors 1S of both these sharks and of the higher 

 mammals, we should be little, if any, nearer to an explana- 

 tion of the phenomenon by means of " Natural Selection," 

 for in the sharks in question the vascular prominences are 

 developed from one fcetal structure (the umbilical vesicle), 

 while in the the higher mammals they are developed from 

 quite another part, viz., the allantois. 



So great, however, is the number of similar, but ap- 

 parently independent structures, that we suffer from a per- 

 fect embarras de richesses. Thus, for example, we have 

 the convoluted windpipe of the sloth, reminding us of the 

 condition of the windpipe in birds ; and in another mammal, 

 13 A view recently propounded by Kowalewsky. 



