68 STRUGGLE FOR EXIBTENVE. 



being over another. Nevertheless, so profound is our 

 ignorance, and so high our presumption, that we marvel 

 when we hear of the extinction of an organic being; and 

 as we do not see the cause, we invoke cataclysms to deso- 

 late the world, or invent laws on the duration of the forms 

 of life! 



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 

 folgens is never visited in my garden by insects, and con- 

 sequently, 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 

 fertilize them. I find from experiments that humble-bees 

 are almost indispensable to the fertilization of the hearts- 

 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 twenty 

 heads of Dutch clover (Trifolium repens) yielded 2,290 

 seeds, but twenty other heads, protected from bees, produced 

 not one. Again, 100 heads of red clover (T. pratense) 



Eroduced 2,700 seeds, but the same number of protected 

 eads 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 heartsease 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 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 elsewhere, which 

 I attribute to the number of cats that destroy the mice." 



