' THE ORIGIN OF SPECIES'" 101 



young fir-trees. Yet who would ever have supposed 

 beforehand that the mere presence or absence of cattle 

 would absolutely have determined the very existence of 

 the Scotch fir throughout a wide range of well-adapted 

 sandy English upland ? 



To take another curious instance mentioned by 

 Darwin. In Paraguay, unlike the greater part of 

 neighbouring South America, neither horses nor cattle 

 have ever run wild. This is due to the presence of a 

 parasitic fly, which lays its eggs in their bodies when first 

 born, the maggots killing off the tender young in their 

 first stages. But if any cause were to alter the number 

 of the dangerous flies, then cattle and wild horses would 

 abound ; and this would alter the vegetation, as Darwin 

 himself observed in other parts of America ; and the 

 change in the vegetation would affect the insects ; and 

 that again the insectivorous birds ; and so on in ever 

 widening circles of incalculable complexity. Once 

 more, to quote the most famous instance of all, the 

 visits of humble-bees are absolutely necessary in order 

 to place the pollen in the right position for setting the 

 seeds of purple clover. Heads from which Darwin 

 excluded the bees produced no seeds at all. Hence, if 

 humble-bees became extinct in England, the red clover, 

 too, would die off: and indeed, in New Zealand, where 

 there are no humble-bees, and where the efforts to 

 introduce them for this very purpose have been uni- 

 formly unsuccessful, the clover never sets its seed at 

 all, and fresh stocks have to be imported at great ex- 

 pense every year from Europe. But the number of 

 humble-bees in any district largely depends upon the 

 number of field-mice, which destroy the combs and 

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