THE NUMBERS OF ANIMALS 109 



Australia 70,000 mice were killed in one stackyard in an 

 afternoon ! ^7 



11. In former years great hordes of a small antelope 

 called the springbok or trekbok used to occur periodically 

 in South Africa.^ These appeared at intervals from the 

 region of the Kalahari Desert, and in some of their migra- 

 tions they marched south into the settled districts, doing great 

 damage to crops on their travels. Eye-witnesses of these 

 migrations have described the fantastically large numbers of 

 animals taking part in them. One observer, after careful 

 estimation, thought that there were half a million animals in 

 sight at one moment, and it could be shown that the area 

 covered by the whole migrating horde occupied a space of 

 country one hundred and thirty-eight by fifteen miles. Even 

 though they were not equally dense throughout, there must 

 have been a good many ! Another says : *' One might as 

 well endeavour to describe the mass of a mile-long sand dune 

 by expressing the sum of its grains in cyphers, as to attempt 

 to give the numbers of antelopes forming the living wave that 

 surged across the desert in 1892 and broke like foam against 

 the western granite range. I have stood on an eminence 

 some twenty feet high, far out on the plains, and seen the 

 absolutely level surface, as wide as the eye could reach, covered 

 with resting springbucks, whilst from over the eastern horizon 

 the rising columns of dust told of fresh hosts advancing." ^ 



12. The results of unchecked increase are also seen in a 

 striking way in the big migrations of locusts and butterflies 

 which have been recorded in various parts of the world. 

 (In some of these cases the unusually large numbers have 

 probably been due to local concentration into migratory 

 swarms, rather than to over-increase of the population by 

 breeding. In the majority of cases, however, there is almost 

 certainly over-population, caused either by excessive increase 

 of animals or by unusual scarcity of food, etc.) 



13. There are numerous cases in which similar outbursts 

 in numbers among still smaller animals have been sufficiently 

 large to attract notice. In Switzerland the railway trains are 

 said to have been held up on one occasion by swarms of 



