336 A. Winchell on the Prairies of the Mississippi Valley. 
3. Diluvial deposits, on the contrary, are found everywhere re- 
plete with living germs. 
which the sudden appearance of unwonted species occurs. 
1st. When a change is produced in the physical condition of 
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germs must have previously existed in the soil. 
2d. When a change is produced in the chemical nature of the 
soil. Illustrations are familiar to every agriculturist. How 
pes must have lain dormant in the soil at a greater or 
