OLD MAIDS AND EOAST BEEF. 259 



shown by experiments, that red clover which is not visited 

 by humming-bees does not yield a single seed. The number 

 of bees is determined by the number of their enemies, the 

 most destructive of which are the field-mice. The more the 

 field-mice predominate, the less the clover is fructified. The 

 number of field-mice, again, is dependent upon the number 

 of their enemies, principally cats. Hence in the neighbour- 

 hood of villages and towns, where many cats are kept, there 

 are plenty of bees. A great number of cats, therefore, is evi- 

 dently of gi'eat advantage for the fructification of clover. 

 This example may be followed stiU. further, as has been done 

 by Carl Vogt, if we consider that cattle which feed on red 

 clover are one of the most important foundations of the 

 wealth of England. Englishmen preserve their bodily and 

 mental powers chiefly by making excellent meat — roast beef 

 and beefsteak — their principal food. The English owe the 

 superiority of their brains and minds over those of other 

 nations in a great measure to their excellent meat. But this 

 is clearly indirectly dependent upon the cats, which pursue 

 the mice. We may, with Huxley, even trace the chain of 

 causes to those old maids who cherish and keep cats, and, 

 consequently, are of the greatest importance to the fructifi- 

 cation of the clover and to the prosperity of England. From 

 this example we can see that the further it is traced the 

 wider is the circle of action and of correlation. We can 

 with certainty maintain that there exist a great number of 

 such correlations in every plant and in every animal, only 

 we are not always able to point out and survey their con- 

 catenation as in the last instance. 



Another remarkable example of important correlations is 

 the following, given by Darwin. In Paraguay, there are 



