50 THE EVOLUTION THEORY 



again increased because of the lessened persecution and elimination. 

 In nature the case is not quite so simple, because the fox does not 

 live on hares alone, and the hare is not preyed upon only by the fox ; 

 but the illustration may serve to elucidate the point that a moving 

 equilibrium is maintained between the species of a district, between 

 persecutors and persecuted, in such a way that the number of 

 individuals in the two species is always varying a little up and 

 down, and that each influences the other so that a regulative process 

 results. Throughout periods of considerable length the average 

 remains the same ; that is to say, a normal number is established. 

 This normal strength of population is the mean above and below 

 which the number of individuals is constantly varying. It is, of 

 course, seldom that the mutual influences and regulations are so 

 simple as in the example given ; usually several or even many 

 species interact upon each other, and not beasts of prey and their 

 victims alone, but the most diverse species of animals and plants, 

 which do not stand in any obvious relation to one another at 

 all. Moreover, the physical, and especially the climatic conditions, also 

 cause the normal number of the species to rise and fall. 



The inter-relations between species living together on the same 

 area are so intricate that I should like to give two other illustrations. 

 Let us first take Darwin's famous instance of the fertility of clover, 

 which depends on the number of cats. It is of course only an 

 imaginary one, but the facts it is based upon are quite correct. The 

 number of cats living in a village to a certain extent determines the 

 number of field-mice in the neighbourhood. These again destroy the 

 nests of the humble-bees, which live in holes in the ground, and thus 

 the number of humble-bees depends on that of the field-mice and cats. 

 But the clover must be pollinated by insects if it is to produce fertile 

 seed, and only the humble-bee has a proboscis long enough to effect 

 the pollination. Therefore the quantity of clover- seed annually 

 produced depends on the number of humble-bees, and ultimately upon 

 the number of cats. And, as a matter of fact, humble-bees were in- 

 troduced into New Zealand from England, because without them the 

 clover would produce no fertile seeds. 



On the grassy plains of Paraguay there are no wild cattle and 

 horses, because of the presence of a fly which has a predilection for 

 laying its eggs in the navel of the newly-born calves and foals, with 

 the result that the calves or foals are killed by the emerging maggots. 

 We may reasonably assume that the numerical strength of this fly- 

 species depends on the distribution of insect-eating birds, whose 

 numbers in turn are determined by certain beasts of prey. These 



