142 



THE DOCTRINE OF DESCENT. 



merous as they are, must be habitually checked by some 

 means, probably by other parasitic insects. Hence if 

 certain insectivorous birds were to decrease in Paraguay, 

 the parasitic insects would probably increase, and this 

 would lessen the number of the navel-frequenting flies; 

 then cattle and horses would become feral, and this would 

 certainly greatly alter (as indeed I have observed in parts 

 of South America) the vegetation, and this, again, would 

 largely affect the insects, and this the insectivorous 

 birds, and so on, in ever-increasing circles of com- 

 plexity.'' 



Another example out of Darwin's store is perhaps 

 even more striking. " I find from experiments that hum- 

 ble-bees are almost indispensable to the fertilization of 

 the heartsease (Viola tricolor), for other bees do not visit 

 this flower. I have also found that the' visits of bees are 

 necessary for the fertilization of some kinds of clover; 

 for instance, 20 heads of Dutch clover (Trifolium repens) 

 yielded 2,290 seeds, but 20 other heads, protected from 

 bees, produced not one. Again, 100 heads of red clover 

 (T. pratense) produced 2,700 seeds, but the same number 

 of protected heads produced not a single seed. Humble- 

 bees alone visit red clover, as other bees cannot reach 

 the nectar. It has been suggested that moths may fer- 

 tilize the clovers; but I doubt whether they could do so 

 in the case of the red clover, from their weight not being 

 sufficient to depress the wing-petals. Hence we may 

 infer as highly probable that, if the whole genus of hum- 

 ble-bees became extinct or very rare in England, the 

 heartsease and red clover would become very rare or 

 wholly disappear. The number of humble-bees in any 

 district depends in a great degree on the number of field- 



