

THE STRUGGLE FOR EXISTENCE. 61 



isms have a tendency to increase in geometrical ratio ; 

 consequently their powers of multiplication are prodi- 

 gious. To take an instance quoted by Wallace, a single 

 pair of flies (Musca carnaria) produce 20,000 carrion- 

 eating larvae, which will hatch out into flies ready to re- 

 produce in about a fortnight, giving rise in turn to some 

 200 million hungry larvae. Linnaeus did not exaggerate 

 when he said that a dead horse would be devoured by 

 flies as quickly as by a lion. The fertility of parasites 

 is proverbial : the eggs of tapeworms and other such 

 internal parasitic worms may be counted by the million. 

 Among the vertebrates the fish are the most prolific a 

 single cod can lay over nine million eggs, though of course 

 they may not all be fertilised/ Equally remarkable is the 

 power of reproduction in plants. It has been calculated 

 that a single cholera bacillus would give rise to sixteen 

 hundred trillions of bacilli in a day if propagating freely, 

 forming a solid mass weighing 100 tons. Doubtless, 

 such unrestrained reproduction rarely if ever occurs; 

 but every organism is, so to speak, ready to seize the 

 opportunity of spreading to an indefinite extent and ia 

 trying, as it were, to extend its range, to colonise fresh 

 regions. Every available spot, from the top of the high- 

 est mountain to the lowest depth of the sea, is invaded, 

 every possible mode of life is adopted, as far as adapta- 

 bility will allow. The various organisms are packed as 

 tight as possible, exerting mutual pressure. If any one 

 increases, it is at the expense of some other. So the 

 fauna and flora of an old established district are made up 

 of a multitude of interdigitating species, availing them- 

 selves of all the resources of the region, fitting close to- 

 gether like the stones of a mosaic, and each occupying 

 that place for which it is best suited. Under ordinary 

 conditions a balance is soon struck, a state of unstable 

 equilibrium established, in which the surviving forms 

 have reached an average number of individuals about 

 which they fluctuate within comparatively narrow limits. 



