54 CONTROLLED NATURAL SELECTION 
a species, and competition between two or 
more species ; to the difference between intra- 
and inter-specific struggles. 
In intra-specific struggles, because the indi- 
viduals are very similar, the struggle will be 
prolonged : very small differences will decide 
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Fie. 8. 
life and death. Under these conditions evolu- 
tion will proceed. 
In inter-specific struggles, which take place 
within common environments (when they be- 
come overcrowded) because the individuals 
are not similar in structure, the struggle will 
be soon over. One species is bound to be 
vastly more fit than the other, and will there- 
fore quickly displace it. Can a struggle be 
