TEE STRUGGLE FOR EXISTENCE. 89 



Nearly all our orchidaceous 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, 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 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 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 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 



