Species and Species Change 



123 



species (Jones and Fuller, 1955), in which case each species would 

 be known from only a single small colony. Many insect species each 

 occupy only one or a few adjacent caves, as for example do the 

 ground beetles Horologion speokoites (Valentine, 1932) and 

 Nelsonites jonesi ( Valentine, 1952 ) . The giant sequoia Sequoiaden- 

 dron giganteiim and the redwood Sequom sempervirens occur in 

 narrow strips only a few hundred miles long (Fig. 48). Every in- 

 termediate can be found between these small ranges and the much 

 larger transcontinental and intercontinental ones. 



Fig. 48. Geographic ranges of the giant sequoia Seqiioiadendron gig,anteiim 

 and the redwood Sequoia sempervirens. (After Munns.) 



Although the range of any one species varies markedly in extent, 

 observations over a number of years can be used to predict its limits 

 and fluctuations with reasonable accuracy. This predictability indi- 

 cates that the genetic changes within a species seldom build up 

 sufficiently to change their observed ecological tolerances within 

 short periods of time. Most of the observed exceptions are inferred 

 and concern species introduced into a foreign environment (Elton, 

 1958). These exceptions may represent simply changes in the fre- 

 quency of genes already in the genome rather than the origin and 

 success of new mutant types. 



