ORGANIC EVOLUTION 29 



for a moment doubt that the gill slits of these em- 

 bryos and of the fish represent the same structures. 

 When we look further into the matter we find that 



Gill slits 



Fig. 13. — Human embryo, showing gill slits 

 and aortic arches. {After His, from Marshall.) 



young fish also possess gill slits (fig. 14) — even in 

 young stages in their development. Is it not then 

 more probable that the mammal and bird possess 



^-GUl slits - 



Fig. 14. — Side views of head of embrvo shark, 

 showing gill slits. {After Sedgwick.) 



this stage in their development simply because it 

 has never been lost? Is not this a more reasonable 

 view than to suppose that the gill slits of the em- 

 biyos of the higher forms represent the adult gill 



