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 
