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CLOVER CROP DEPENDS ON SPINSTERS 



The red clover prospers if there are plenty of bumblebees (which are able, however, 

 to thrive on other plants). But bumblebees are destroyed by field mice, which are 

 kept in check by cats. If certain kind ladies did not harbor the cats, the mice might 

 become too numerous and destroy the bumblebees, and we should then not be in 

 clover 



These in turn are devoured by a dozen or a score of small birds. But these 

 birds can barely supply a single pair of hawks or a domestic cat. The hawks 

 or cats do not, of course, finish the cycle, even if they have no serious enemies 

 to contend with. For if there are no enemies large enough to destroy them, 

 the parasitic chain gets in its work sooner or later. Exceptional prosperity 

 leads to high density of population — which invites an epidemic. An epi- 

 demic in turn exhausts itself, or it is destroyed by another epidemic. 



Natural Groupings of Organisms We can separate a plant or an animal 

 from others of the same or of other species. But we cannot keep it alive in 

 isolation indefinitely. The organism depends upon other species in its natural 

 setting — on some directly as food, on others indirectly as food for its prey or 

 host. The cat seems not to care much about clover; but she can feed on 

 field mice only because the clover and the bumblebee have an arrangement 

 of their own. 



When you see a field aglow with fireweed or black-eyed Susan, you may 

 be sure it was the winds of pure chance that brought the seeds. Yet the 

 insects which now fly around those flowers were directed by something more 

 definite. And whatever becomes of the next crop of seeds, these insects will 



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