The Making of Species 
beast of prey and its victim are evenly matched 
as regards fleetness and power of endurance that 
small variations in these qualities can have a 
survival value. But in the rough and tumble 
of the struggle for existence the victim and its 
foe are but rarely well-matched. Take as an 
example the case of a flycatcher. ‘ This bird,” 
writes D. Dewar, ‘will sometimes take three 
or four insects in the course of one flight ; all 
are captured with the same ease, although the 
length of wing in each victim varies. So great 
is the superiority of the bird that it does not 
notice the difference in the flying powers of its 
puny quarry.” It is unnecessary to labour this 
point. 
9. Species or varieties differing considerably 
in colour may exist side by side, as the hooded 
and carrion crows, the white and dark breasted 
forms of the Arctic skua, the pale and dark forms 
of the fulmar petrel, the grey and rufous forms of 
the American scops owl (Megascops asio). 
It is true that preponderance of one form or 
another in certain districts points to some advan- 
tage possessed by one over the other, but, for 
all we know, it may be due to heredity, and in 
any case the co-existence of the two types in part 
of their range, or at certain seasons, shows that 
selection is not at all rigorous. 
The same argument applies to the co-existence 
of very differently-coloured species with generally 
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