The Making of Species 
struggle for existence, in other words, of the 
modus operandi of natural selection. 
The tsetse-fly in Africa is a far more important 
check on the increase of some animals than the 
lions and other beasts of prey. There are in 
that continent large tracts of country, known as 
tsetse-fly belts, in which neither horse, nor ox, 
nor dog can exist. If races of these animals 
were to arise which could withstand the bite of 
the tsetse-fly, these species might increase more 
rapidly than the rabbit in Australia has done, 
nor would it matter if the creatures in question 
were bright crimson, or any other conspicuous 
colour. 
Take the case of the lion in Africa. The chief 
bar to the increase in numbers of this species 
appears to be the teething troubles to which the 
whelps are liable. Now suppose that a mutation 
were to occur in the lion. Suppose that several 
members of a litter were all bright blue, and that 
these suffered from no teething troubles. They 
would probably all grow up, and although at 
some disadvantage as hunters on account of their 
conspicuous colouring, they would nevertheless 
probably increase at the expense of the normally 
coloured lions, because of the immunity of their 
offspring from death from teething troubles. 
Zoologists would then be at a loss to explain 
their bright colouring. We should have all manner 
of ingenious suggestions raised, namely, that in the 
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