NUMBER OF ORGANISMS CONSTANT. 255 



by a few examples the operation of the struggle for life, and 

 the operation of natural selection by means of the struggle 

 for life (Gen. Morph. ii. 231). 



When considering the struggle for life, we started from 

 the fact that the number of germs which all animals and 

 plants produce is infinitely greater than the number of 

 individuals which actually come to life and remain alive 

 for a longer or shorter time. Most organisms produce 

 during life thousands or millions of germs, from each of 

 which, under favourable circumstances, a new individual 

 might arise. In most animals and plants these germs are 

 eggs, that is cells, which for their development require 

 sexual fructification. But among the Protista, the lowest 

 organisms, which are neither animals nor plants, and which 

 propagate themselves only in a non-sexual manner, the germ- 

 cells, or spores, require no fructification. Now, in all cases 

 the number of unsexual, as well as of sexual germs, is out 

 of all proportion to the number of actually living indi- 

 viduals of every species. 



Taken as a whole, the number of living animals and plants 

 on our earth remains always about the same. The number 

 of places in the economy of nature is limited, and in most 

 parts of the earth's surface these places are always approxi- 

 mately occupied. Certainly there occur everywhere and in 

 every year fluctuations in the absolute and in the relative 

 number of individuals of all species. However, taken as a 

 whole, these fluctuations are of little importance, and it is 

 broadly the fact that the total number of all individuals 

 remains, on an average, almost constant. There is a 

 constant fluctuation, which depends on the fact that in one 

 year or another one or other series of animals and plants 



