30 EVOLUTION AND GENETICS 



slits of the fish that in some mysterious Avay have 

 been pushed back into the embryo of the bird? 



Many similar examples con Id be given. All can 

 be interpreted as embryonic survivals rather than as 

 phyletic contractions. Not one of them calls for the 

 latter interpretation. 



The stud}" of the cleavage i^attern of the segment- 

 ing egg furnishes the most convincing evidence that 

 a different explanation from the one stated in the 

 biogenetic law is the more probable explanation. 



It has been found that the cleavage pattern has 

 the same general arrangement in the early stages of 

 flat worms, annelids and molluscs (fig. 15). Ob- 

 viously these stages have never been adult ancestors, 

 and obviouslv if their resemblance has any historical 

 meaning at all, it is that each group has retained the 

 same general plan of cleavage, possessed by their 

 common ancestor. 



Accepting this view, does the evidence from em- 

 brvoloffv favor the theory of evolution? I think that 

 it does very strongly. The embryos of the mammal, 

 bird, and lizard have gill slits today because gill slits 

 were present in the embryos of their common ances- 

 tors. There is no other view that explains so well 

 their presence in the higher forms. 



It may be asked whether this is not all that the 

 "biogenetic law"claims. Has not the old conclusion 

 been reached in a roundabout wav? I think not. To 

 my mind there is a wide difference between the old 



