iv] NUMBERS AND DENSITY OF SPECIES 71 



by leaps and bounds and the whole habitable world 

 would soon be choked. We do not know the factors 

 which keep down the numbers of such powerful and 

 sagacious birds as macaws. The osprey is one of 

 the most cosmopolitan birds of prey ; it is stronger 

 than most other fish-eating birds, and there are surely 

 enough fish in the rivers and lakes for more osprey s, 

 yet they are nowhere common. 



We know that over-crowding, over-population of 

 a district, engenders diseases, or rather let us say 

 that these diseases are always present somewhere, 

 comparatively harmless if kept going by a chance 

 victim, but that they spread in a crowd. If a species 

 has become so numerous that there is practically 

 contact between its members throughout its range, 

 the whole may be carried off. Only those which 

 happen to live in some isolated district, cut oif by 

 chance, will be saved and, like the animals disembarking 

 from the Ark, start afresh. The degree of the density 

 of population necessary to produce this 'contact' we 

 do not know. Countries with locusts and Bombycidae 

 or spinning moths are also inhabited by ichneumon 

 wasps. They balance each other, but when there 

 occurs one of the great irruptions into the North 

 German pine forests by the moth Gastropacha pini, 

 the caterpillars of which then devastate whole forests, 

 in the second or third year the ichneumons have also 

 become so abundant that every caterpillar is attacked 



