STRUGGLE FOR EXISTENCE 113 



quire the visits of insects to remove their pollen-masses 

 and thus to fertilize them. I find from experiments that 

 humble-bees are almost indispensable to the fertilization 

 of the heart's-ease (Violo 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 

 fertilize 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 heart's-ease and red clover would become very rare, 

 or wholly disappear. The number of humble-bees in any / 

 district depends in a great measure upon the number of 

 field-mice, which destroy their combs and nests; and Col. 

 Newman, w^ho has long attended to the habits of humble- 

 bees, believes that "more than two-thirds of them are 

 thus destroyed all over England." Now the number of 

 mice is largely dependent, as every one knows, on the 

 number of cats; and Col. Newman says, "Near villages 

 and small towns I have found the nests of humble-bees 

 more numerous than elsewhere, which I attribute to the 

 number of cats that destroy the mice." Hence it is quite 

 credible that the presence of a feline animal in large 

 numbers in a district might determine, through the in- 



