132 THE WONDER OF LIFE 



space of something like a quarter of a million cubic feet, 

 allowing 200,000 flies to a cubic foot. An oyster may have 

 sixty million eggs, and the average American yield is 

 sixteen millions. If all the progeny of one oyster survived 

 and multiplied, and so on till there were great-great-grand- 

 children, these would number sixty-six with thirty-three 

 noughts after it, and the heap of shells would be eight times 

 the size of the earth ! Of course none of these things 

 happen, because of the checks imposed by the struggle for 

 existence. Yet every now and then, as man knows to his 

 cost, a removal or diminution of the natural checks allows 

 the potential productivity to assert itself for a short time 

 or within a limited area. The river of life sometimes does 

 overflow its banks, as it always tends to do, and the result- 

 ing flood is called a plague. But one plague brings another 

 in its train, as in Egypt long ago, and things right them- 

 selves, usually with considerable loss in the process. 



The large African land-snail Achatina fulica was intro- 

 duced about 1900 into central Ceylon, but was shortly after- 

 wards practically exterminated. A couple that escaped 

 destruction were carried down some years afterwards to the 

 low country. ' Here they increased to such an amazing 

 extent, over an area of about five square miles, that 

 their numbers were to be reckoned by millions, no fewer 

 than 227 being counted in a cluster on the stem of a 

 cocoa-nut palm in a length of about 6 feet '. Luckily little 

 or no damage has been done, as the snail acts as a scavenger. 

 The adults are attacked by a terrapin of the genus Nicoria, 

 the young stages have many enemies, and the early 

 exuberance of multiplication is now being checked. 



On the night before the new or full moon in the middle 

 or latter half of December there occurs the remarkable 



