568 APPLIED BIOLOGY 



and plants under domestication. It is often called artificial 

 selection to distinguish from natural selection ( 500). 



499. Struggle for Existence. Long before Darwin wrote 

 the " Origin of Species " (1859) it was recognized by scien- 

 tific men that vastly more individual animals and plants came 

 into the world than can possibly survive, for there is not room 

 and food for them all. To illustrate : Probably not one oyster 

 embryo in a million grows to maturity; and if none of them 

 perished for a very few generations, the oceans would be 

 solid with oysters. 



Elephants multiply slower than all other animals, but if 

 all elephants lived one hundred years and produced but six 

 young per pair, there would be after 800 years about 19,000,- 

 000 living descendants of one pair. Imagine each of these 

 descendants (9,500,000 pairs) reproducing at the same rate 

 for another 800 years ! And yet elephants have been on this 

 earth many times 800 years and living elephants are not very 

 numerous anywhere. Evidently the vast majority do not 

 live long enough to produce six young per pair. In fact, 

 most of them do not live through the 30 years required to 

 reach maturity. 



Among plants, we have only to watch a patch of weeds 

 crowding each other and the cultivated plants in a garden 

 in order to see how severe is the struggle resulting from over- 

 population. Suppose an annual plant this year produces 

 100 seeds, that each of these next year will form a plant with 

 100 seeds, and so on for many years. There would be 100 

 plants next year, 10,000 the second year, 1,000,000 the third 

 year, and so on in geometrical ratio multiplied by 100 each 

 year. Evidently the world would soon be full of that kind 

 of plant. But although many plants produce far more than 

 100 seeds annually, none of them increase at such a rate. 

 Obviously, the vast majority of seeds do not develop into 

 mature plants. Only a very small proportion of the sum 

 total of all kinds of seeds formed in any one year could ever 



