98 



Natural Selection 



characters. Through the fossil record it is possible to obtain some 

 understanding of the long-term results of natural selection (Fig. 

 40). Some hneages living today have existed with little change for 



oo 



< < 

 -1 z 



3 < 



a a. 



5 < 1^^005 



O i (E I o< 



Z K o t ^<E 



Ij t- < in o o 



I- - z 



(c a: uj 



O I- Q. 



Fig. 40. An example of the results of natural selection. The range and de- 

 velopment in time of the superfamilies of the animal phylum Brachiopoda, 

 illustrating the differences in taxonomic size, time of "flowering," and patterns 

 of survival. The thinnest line drawn (for example, Lingulacea from Pennsyl- 

 vanian to Tertiary times) equals one genus. (After Cooper and Williams.) 



many millions of years— the American opossum for possibly 100 

 million and many marine clams for possibly 350 million. Other 

 existing lineages, such as that which eventually evolved into the 

 horse Equus, have been changing fairly steadily for the last 100 

 million years. Their primitive ancestral forms are long extinct. In 

 Equus it appears that change is synonymous with survival, but this 

 is not universally true of other lineages. The reptilian dinosaurs and 



