Natural Selection; Origin of Species - 547 



Fig. 28-6. Origin and extinction of varieties and species by natural selection. 

 The horizontal dimension represents time (i.e., the succession of generations); 

 the vertical dimension, variation. The very short branches represent the numer- 

 ous mutations that are quickly eliminated by natural selection; the longer 

 branches, those that persist for longer periods. Letters at left indicate original 

 species; those at right, surviving species. (Adapted from Darwin.) 



intervals between the successive vertical lines 

 in the figure may be taken to indicate ap- 

 proximately a thousand generations. Each 

 species continues to produce new mutant 

 varieties, and although most of these vari- 

 eties are quickly eliminated by natural selec- 

 tion, some persist for longer periods. Thus, in 



the course of 10,000 generations, the several 

 original species may meet with totally differ- 

 ent fates, as may be seen in the figure. One 

 species has given rise to several new forms, 

 while the original species has disappeared; in 

 another case, the original species has per- 

 sisted without changing, while all its off- 



