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 Para- 

 guay, 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-increas- 

 ing circles of complexity." 



Another example out of Darwin's store is perhaps even 

 more striking. " I find from experiments that humble- 

 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 weiq"ht not beinG: 

 sufficient to depress the wing-petals. Hence we may 

 infer as highly probable that, if the whole genus of 

 humble-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 



