OLD MAIDS AND ROAST 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 
evidently of great advantage for the fructification of clover. 
This example may be followed still 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 
1, ea 
