EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 31 



ers unacquainted with details of vertebrate anatomy. Actually, however, 

 our illustrative material might have been drawn from any portion of the 

 body. All systems and parts of the bodies of vertebrates exhibit the funda- 

 mental similarities which we have designated as homologous. Thus, the 

 skulls of vertebrates have received exhaustive investigation. Studies re- 

 veal that "from fish to man" a common pattern of bone arrangement is 



premaxilloiry. 

 maxillary 

 acrimod 



orbiH 



squotmosod 

 MAMMALIA 



AMPHIBIA 



bones not found in 

 moimmalian skull 



FIG. 3.6. Basic structural plan of the roof bones of the skull in amphibians, reptiles, and 

 mammals. Bones present in mammals are named, others are shown in solid black. (After 

 Zangerl, "The methods of comparative anatomy and its contribution to the study of 

 evolution," Evolution, Vol. 2, 1948.) 



found; evolution has consisted of gradual reduction in numbers of bones, 

 through loss and through fusion of one bone with another, and of changes 

 in function and in relative size. Fig. 3.6 illustrates the point that the skulls 

 of amphibians, reptiles, and mammals are based upon this common pat- 

 tern. The figure also demonstrates progressive reduction in number of 

 bones and the corresponding increase in importance of such bones as the 

 frontals and parietals as the brain underlying them increases in size. Why 

 do skulls of such diverse animals give evidence of having been constructed 

 on a common pattern? Because, if the evolutionary interpretation is cor- 

 rect, the diverse animals all inherited that pattern from a common an- 

 cestor. 



