THEORY OF EVOLUTION 



CJ-] 



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 tlian to suppose 

 that the gill slits of the embryos of the higher 

 forms represent tlie adult gill slits of the fish 



Fig. 11. Side views of iiead of emI)ryo sharks, showing gill 

 slits. (After Sedgwick.) 



that in some mysterious way have been pushed 

 back into the embryo of the l)ird? 



I could give many similar examples. All 

 can be interpreted as embryonic survivals 

 rather than as phyletic contractions. Xot 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. 



