vii] PLANT POPULATION 105 



may be taken as a unit, and considered among the 

 millions of like areas which go to form a country-side. 

 What is the probability of the light grain of some 

 grass, say of Holcus, Fescue, or of the Trembling 

 Grass, with its chaffy paleae serving as sails, being 

 carried by the wind, and finally coming to rest, till 

 under favourable circumstances it shall germinate? 

 Obviously the chances are very remote, but equally 

 clear is it that the larger the number of germs 

 produced the greater will be the probability. We 

 shall then enquire with interest into the fecundity of 

 plants, as estimated by the number of germs they 

 produce in a given time ; for plainly this is a factor 

 which will go far in determining the plant-population as 

 we see it established in any area open to the free play 

 of natural conditions. 



The third Chapter of Darwin's Origin of Species 

 dealt with the geometrical ratio of increase of living 

 things. Following Malthus he there pointed out that 

 " there is no exception to the rule that every organic 

 being naturally increases at so high a rate, that, if 

 not destroyed, the earth would soon be covered by 

 the progeny of a single pair." The effect of a 

 geometrical ratio of increase is apt to be under- 

 estimated ; but a simple example makes it clear. If 

 a plant produces 10 seeds each year, and each of 

 these germinates, the result in six successive years 

 will be 10, 100, 1000, 10,000, 100,000, 1,000,000. 



