THEORY OF EVOLUTION 21 



then more probable that the mammal and 

 bird possess 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 embryos of the higher 

 forms represent the adult gill slits of the fish 



FIG. 11. Side views of head of embryo sharks, showing gill 

 slits. (After Sedgwi c k.) 



that in some mysterious way have been pushed 

 back into the embryo of the bird? 



I could give many similar examples. All 

 can be interpreted as embryonic survivals 

 rather than as phyletic contractions. Not one 

 of them calls for the latter interpretation. 



The study of the cleavage pattern of the 

 segmenting egg furnishes the most convincing 

 evidence that a different explanation from the 

 one stated in the biogenetic law is the more 

 probable explanation. 



