34 NATURAL HISTORY OF VERTEBRATE ANIMALS 



nutritional relationship, upon which have developed all the mul- 

 titudinous interactions and special devices by which animals and 

 plants maintain themselves upon our planet. 



Inter-relations between Species. — In concluding this discus- 

 sion of the vertebrates in relation to their environment, reference 

 may be made to the manner in which both animals and plants are 

 inter-related in their daily existence. It is well known that ani- 

 mals depend upon other animals and upon plants for food. The 

 relation of '' eater " and " eaten " is almost universal. A host of 

 other relationships also obtain, whereby species depend upon other 

 species for their very lives; and the chain of events often ramifies 

 in so many directions that we can only imagine its ultimate possi- 

 bilities. Among many examples of such relationships are those 

 cited by Darwin in discussing the " checks to increase " in his 

 " Origin of Species." The relation between cats and clover is 

 one of his most famous cases. Stated in Darwin's own language, 

 this runs as follows : 



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 2290 seeds, but 20 other 

 heads protected from bees produced not one. Again, 100 heads of 

 red clover ( T. pratense) produced 2700 seeds, but the same niunber 

 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 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 hmnble- 

 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 Col. 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 Col. 

 Newman says, " Near villages and small towns I have found nests 

 of humble-bees more numerous than elsewhere, which I attribute 

 to the nmnber 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. 

 (Darwin, Chas., " Origin of Species," pp. 90-91.) 



