92 THE GEXESIS OF SPECIES. [Chap. 



indeed it might be supposed to end ; Ijut, strange as it 

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

 certain of the ascidians, sometimes called tunicaries or 

 sea-squirts. 



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

 connnon ancestors* 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 " Xatural Selection ; " 

 for in the sharks in question the vascular prominences are 

 developed from one fcetal structure (the umbilical vesicle), 

 while in the higher mammals they are develoj^ed from 

 quite another part, viz. the allantois. 



So great, however, is the nund)er of similar, but ap- 

 parently independent, stnictures, that we suffer from an 

 emharras dc richcsscs. 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 tlie highest apes 

 the caecum 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, niav be adduced 



1 A view ])ropoiuule(l by Kowak'wsky, and since supported liy KupllVr, 

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

 see his article entitle*! "Ueber die so^'cnannte Cliorda der Ascidieiilarvon 

 und die Vernieiiitlithe Verwaniltsihalt von Wirbellosen und Wirbel- 

 tliieren," in Ileichcrt and KeyniuiKl's "Arehiv fiir Auatoude uud 1'liy.si- 

 ologie," 1870, No. 6, page 701. 



