130 



Species and Species Change 



total and regional oscillations to approach the random character 

 described by Cole (1951). 



From the standpoint of understanding genetic changes in an 

 entire species, it is necessary to know whether certain areas of the 

 range have habitual high population densities from which indi- 

 viduals and therefore also genetic variability flow into areas of low 

 density or whether some other pattern prevails. An extremely large 

 body of recorded data suggests that almost every conceivable pat- 

 tern occurs and that these patterns are governed by ecological 

 factors and the species vagility. In the example previously cited of 

 the high plains grasshopper Dissosteira longipennis (Fig. 50), the 

 general trend of individual movement is almost certainly from the 

 small and fairly stable minimum range outward (Wakeland, 1958). 

 In the African migratory locusts Locusta migratoria migratorioides 

 (Fig. 51) and Schistocerca gregaria, high population densities de- 

 velop in widely scattered small areas and flow out over a tremen- 

 dous total range (Uvarov, 1951). 



r^''"^», 1953 



Fig. 53. Areas of high population density of the wheat stem sawfly Cephas 

 cinctus during the years between 1953 and 1957 in the Canadian portion of 

 its range. (Adapted from Farstadt.) 



