142 TIE 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 nota 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 
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 
