134 Species and Species Change 



Frye and Leonard, 1952, 1957; P. Smith, 1957). The present biota 

 of previously glaciated terrain represents such range shifting. 



The Wisconsin ice sheet in North America seems to have melted 

 back moderately rapidly, geologically speaking, yet in the last 

 8,500 years its edge has moved north little over 2,000 miles. This 

 averages one fourth of a mile per year. Undoubtedly in some years 

 the retreat was more rapid, and numerous readvances increased the 

 total mileage covered. Although it is possible that during certain 

 periods the ice margin melted back as much as a mile per year, 

 organisms following in its wake would need to have averaged only 

 a fourth of a mile a year ( 440 yards ) to keep up with glacial regres- 

 sion. Presumably average climatic conditions moved at the same 

 tempo. Compared with local and annual differences observed in 

 any one place today, these small changes would have been im- 

 perceptible from year to year along the ice margins and the move- 

 ment of the range would have been an extremely slow process. 



The mechanics of the range movements were unquestionably 

 like those observed today. In times of northward movement, the 

 populations on the extreme southern edge of the range of cool- 

 adapted species would be reduced by adverse conditions at a rate 

 greater than counter fluctuations would permit recolonization of 

 depleted southern areas; the populations on the extreme northern 

 edge of the range would be able to colonize new areas to an extent 

 greater than adverse cycles cut them back. The populations in the 

 middle would fluctuate in the same fashion that they do today. 

 In other words, by amoeboid oscillations of peripheral populations, 

 the range of a species gradually follows the movement of the 

 ecological conditions to which the species is adapted. 



In this context size of present range becomes an ephemeral char- 

 acter. A species called "successful" because today it is widespread 

 and abundant may simply be inhabiting an unusually large acreage 

 having ecological conditions to which it is adapted; in the geological 

 tomorrow the area over which these conditions prevail may be 

 much smaller and the range of the species correspondingly reduced. 

 Yet, in its own habitat, the species would be no less "successful." 

 The abundant species of migratory ducks and geese nesting in 

 the marshes and tundra of North America may now have available 

 a greater area of suitable habitat than ever before. It is entirely 

 possible that, during the maximum extent of Wisconsin glaciation, 

 areas suitable for them were small, and the total populations of 

 these species accordingly were small. Fiuthermore, it is possible that 

 when the ice recedes even more than now their present range may 



