THE STRUGGLE FOR EXISTENCE. 89 



p ^ Nearly all our orcMdaceoas plants abso- 



lutely require the visits of insects to remove 

 their pollen-masses and thus to fertilize them. I find 

 from experiments that humble-bees are almost indispensa- 

 ble to the fertilization of the heart's-ease ( 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, 30 heads of Dutch 

 clover {Trifolium repens) yielded 3,390 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 can not 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 suflBcient 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 on the number of 

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

 Colonel Newman, who 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 Colonel Newman 

 says, "Near villages and small towns I have found 

 the nests of humble-bees more numerous than else- 

 where, 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 intervention first of mice 



