Species and Species Change 131 



In many other species local or regional population peaks appear 

 irregularly over the entire range, and there appears to be little 

 movement from such peak areas into less densely populated sur- 

 rounding areas. This situation is exemplified by the high density 

 of the North American wheat sawfly Cephiis cinctus in the Canadian 

 portion of its range between 1953 and 1957 (Fig. 53) (Farstadt, 

 1953-1957). In 1953 these high density areas formed seven small- 

 to medium-sized areas roughly arranged along a wide arc. In 1954 

 the high density areas formed two large, one medium, and one 

 minute area, of which the southern large one was displaced con- 

 siderably westward from the two southeastern highs of the preced- 

 ing year. In 1955 there were only six small high areas, arranged 

 somewhat along the arc of the 1953 series; the large southern and 

 eastern highs of 1953 and 1954 had dropped to lower population 

 levels. In 1956 high populations appeared in only one small area 

 which dropped to a lower level in 1957. In this latter year, high 

 populations appeared in only four minute southern areas. Because 

 the wheat sawfly is non-migratory, these data indicate that the pop- 

 ulation level of each area rose and fell independently of surround- 

 ing areas. This point is strengthened by an examination of the 

 population density maps for 1953 and 1954. In 1953 (Fig. 54a), 

 the total area of heavy infestation (high population density) was 

 small, but the area of light infestation was large; in 1954 (Fig. 

 54b), the "heavy" areas were much larger, but the northwestern 

 and extreme eastern "hght" areas were at a lower population level. 

 Obviously this series of population changes is not a build up in the 

 middle accompanied by a radial expansion but was caused by den- 

 sity changes occurring independently in each area. These changes 

 occur in an area having habitual kaleidoscopic fluctuations in 

 climate. 



Shift in Geographic Position 



During the past, climates have changed repeatedly in difterent 

 parts of the world. New ranges of mountains, for example, have 

 caused xeric conditions over previously wetter areas and have re- 

 stricted mesic situations to local pockets such as ravines. Climatic 

 bands may shift. Seas may retreat, uncovering large new areas for 

 terrestrial living, or seas may advance, supplanting previous ter- 

 restrial habitats with new areas available for marine life. 



Species ranges have shifted with the climatic or other ecological 

 changes. The movements of these ranges in geologic time took place 



