68 ON THE ORIGIN OF SPECIES 



not see the cause, we invoke cataclysms to desolate 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, most 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, in this part of England, is never visited 

 by insects, and consequently, from its peculiar structure, 

 never can set a seed. Many of our orchidaceous plants 

 absolutely require the visits of moths to remove their 

 pollen-masses and thus to fertilise them. 1 have, also, 

 reason to believe that humble-bees are indispensable to 

 the fertilisation of the heartsease (Viola tricolor), for 

 other bees do not visit this flower. From experiments 

 which J have lately tried, I have found that the visits 

 of bees are necessary for the fertilisation of some kinds 

 of clover ; but humble-bees alone visit the red clover 

 (Trifolium pratense), as other bees cannot reach the 

 nectar. Hence I have very little doubt, 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 

 degree on the number of field-mice, which destroy 

 their combs and nests ; and Mr. H. 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 Mr. 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 intervention 

 first of mice and then of bees, the frequency of certain 

 flowers in that district ! 



In the case of every species, many different checks, 

 acting at different periods of life, and during different 

 seasons or years, probably come into play ; some one 



