DARWIN has shown how die extermination of cats could affect the growth of clover. He writes : ‘I find from experience that humble-bees are almost indispensable to the fertilisation of the heartsease (Viola 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, twenty heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but twenty other heads protected from bees produced not one. Again, a hundred heads of red clover (Trifolium 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. . . . 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 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 everyone knows, on the number of cats, and 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 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.’
Other aspects and problems of field natural history, of the kind mentioned above are malaria infection carried by the bites of mosquitoes and sleeping sickness spread by the bites of the tsetse flies; the activities of earthworms in the soil; the activities of animals and insects that are enemies of crops, timber and other resources; the conservation of game and of marine fisheries; and the control of numerous insect pests.
Among the social insects many excellent examples of animal behaviour may be found, and there are also many observations and experiments which have been carried out and put on record by numerous students working on these creatures. The communities of the social insects, especially of the ants, termites, bees and wasps, also show similarities in various respects to the society of mankind.