470 



ESSENTIALS OF ZOOLOGY 



Relative Numbers of Ten Most Abundant Insects From Overgrazed and 

 Normal Grassland. (Wichita Mountains Wildlife Refuge.) 



GENUS 



common NAME 



normal 



OVERGRAZED 



Seasonal Changes 



It is a matter of common observation that the animals observed 

 in any one place differ greatly from season to season during the 



Fig. 203. — Fluctuations in populations of four Protozoa in an artificial lake in 



Oklahoma during the months of the year, 



year. This is not only true of those animals which, like many birds, 

 migrate southward on the approach of winter and return to north- 

 ern climates for nesting. Some animals hibernate or aestivate, and 



