90 STRUGGLE FOR EXISTENCE. [Chap. HI. 



I am tempted to give one more instance showing 

 how plants and animals remote in the scale of nature, 

 are bound together by a web of complex relations. I 

 shall hereafter have occasion to show that the exotic 

 Lobelia fulgens is never visited in my garden by in- 

 sects, and consequently, from its peculiar structure, 

 never sets a seed. Nearly all our orchidaceous plants - 

 absolutely require the visits of insects to remove their 

 pollen-masses and thus to fertilise them. I find from' 

 experiments that humble-bees are almost indispensable 

 to the fertilisation of the heartsease (Violo tricolor), for 

 other bees do not visit this flower. I have also found 

 that the visits of bees are necessary for the fertilisation 

 of some kinds of clover; for instance, 20 heads of Dutch 

 clover (Trifolium repens) yielded 3,390 seeds, but 30 

 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 fertilise the clovers; 

 but I doubt whether they could do so in the case of the 

 red clover, from their weight not being sufEcient to 

 depress the wing petals. Hence we may infer as highly 

 probable that, if the whole genus of humble-bees be- 

 came extinct or very rare in England, the heartsease 

 and red clover would become very rare, or wholly dis- 

 appear. 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, who has long attended to the habits of hum- 

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

 are thus destroyed all over England." Now the num- 



