28 EVOLUTION AND GENETICS 



with the statement of the biogenetic "law"; for 

 actual experience with discontinuous variation has 

 taught us that new characters that arise do not add 

 themselves to the end of the line of already existing 

 characters but if they involve adult characters they 

 change them without, as it were, passing through 

 and bevond them. 



I venture to think that these new ideas and this 

 new evidence have played havoc with the biogenetic 

 "law." Nevertheless, there is an interpretation of 

 the facts that is entirely compatible with the theoiy 

 of evolution. Let me illustrate this by an example. 



Fig. 12. — Diagram of head of chick, A and B, showing 

 gill slits and aortic arches; and head of fish, C, showing 

 aortic arches. {After Hesse.) 



The embryos of the chick (fig. 12) and of man 

 (fig. 13) possess at an early stage in their development 

 gill slits on the sides of the neck like those of fishes. 

 No one familiar with the relations of the parts will 



