EVOLUTION AND HEREDITY 



345 



its limb buds are somewhat like fins, yet such facts do not 

 necessarily mean that mammals were fishes at one stage in 

 their racial development. 



Though the ontogeny of a particular animal usually 

 repeats the general sequence of descent, the record may be 

 warped or distorted by events which did not occur in racial 

 development, or it may fail to agree in other respects. 

 There are cases of " larval adaptation" in which a develop- 



fish 



So/amander 



Frog 



Turtle 



Chicken 



Man 



FIG. 115. Showing stages in the development of vertebrates. The fish and 

 salamander have functional gills in the last stage, but all other animals repre- 

 sented lose them in earlier stages. 



ing animal possesses certain structures (placenta of mam- 

 mals, Fig. 107; anal filament of young crayfish, Fig. 30) 

 which could not have been present in mature free living 

 ancestors when in a comparable stage of evolution. In 

 other instances certain stages which probably existed in 

 ancestral types have been dropped out. In such " accelera- 

 tion in development" the record is not complete. It is 



