SOME SOURCES OF OUR FAUNA AND FLORA 297 



The driving power back of these movements is the urge of 

 rapidly multiplying life. Each species tends to fill its habitat 

 full to overflowing. So prolific are living things that the pressure 

 to move out into adjacent territory is only checked by impassable 

 barriers that raise the swelHng tide until some outlet is found or 

 multiplication is offset by the high mortality of keen competi- 

 tion. So quiet and unobtrusive is the life of the ordinary plant 

 and animal that we humans find it difficult to realize how keen 

 is the struggle for existence or with what celerity any new 

 territory is occupied. There is no roar of hostile guns along the 

 battle-line of opposing animal and plant interests, no press 

 dispatches tell of starving thousands, no blare of trumpets or 

 floating banners mark the progress of quiet invasion. But there 

 is, nevertheless, tremendous activity. Reproduction is at a 

 geometrical rate; that is, a pair does not give rise to another 

 pair merely, but to a litter, a swarm, a horde. A single female 

 codfish lays five milHon eggs at one spawning. A pair of potato 

 beetles would produce in one season a progeny of sixty million, 

 if all their offspring and their offspring, too, lived unmolested. 

 ''The descendants of a common housefly would in the same 

 time — six generations of about three weeks each — occupy a space 

 of something Hke a quarter of a million cubic feet, allowing two 

 hundred thousand 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 multi- 

 plied, and so on until there were great-great-grandchildren, these 

 would number sixty-six with thirty-three noughts after it, and 

 the heap of shehs 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 aUows the potential productivity to assert itself 

 for a short time or within a hmited area. The river of life some- 

 times does overflow its banks, as it always tends to do, and the 

 resulting flood is called a plague '' (Thomson, The Wonder of Life) . 



