546 - Heredity and Evolution 



Fig. 28-5. Land areas of Western Europe in the third interglacial stage. 

 Present areas, in black; former areas, lighter. (From Osborn, Men of the Old 

 Stone Age. Permission of Charles Scribner's Sons.) 



tions of one group cannot spread over into 

 the other. Continually and gradually the 

 stocks become very different, and finally a 

 genetic incompatibility will result. Thus, 

 when interbreeding becomes impossible, the 

 isolation is complete, and the further evolu- 

 tion of the stocks must be as separate species. 



Natural selection, acting upon a group of 

 closely related organisms, over a long period 

 of time, inevitably produces results such as 

 are shown in Figure 28-6. The letters at the 

 left represent a group of seven closely related 

 species or — starting one step earlier — seven 

 isolated varieties of a single species; and the 



