92 THE GENESIS OF SPECIES. [CHAP. 



indeed it might be supposed to end ; but, strange 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 1 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 foetal structure (the umbilical vesicle), 

 while in 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 an 

 enibarras de richesses. Thus, for example, we have the 

 convoluted windpipe of the sloth, reminding us of a con- 

 dition of the windpipe met with in birds ; and in another 

 mammal, allied to the sloth, namely the great ant-eater 

 (Myrmecophaga), we have again an ornithic character in 

 its gizzard-like stomach. In man and the highest apes 

 the ca3cum has a vermiform appendix, as it has also in 

 the wombat! 



Also the similar forms presented by the crowns of the 

 teeth in some seals, in certain sharks, and in some extinct 

 Cetacea may be referred to ; as also the similarity of the 

 beak in birds, in some reptiles, in the tadpole, and in 

 cuttle-fishes. As to entire external form, may be adduced 



i A view propounded by Kowalewsky, and since supported by Kupffer, 

 but which has quite recently been strongly controverted by W. Donitz : 

 see his article entitled " Ueber die sogenannte Chorda der Ascidienlarven 

 und die Vermeintliche Verwandtschaft von Wirbellosen und Wirbel- 

 thieren," in Reichert and Reymond's "Archiv fur Anatomie und Physi- 

 ologic," 1870, No. 6, page 761. 



