The
same rule applies to almost all male birds; and these battles have been
observed in such different groups as humming-birds, finches,
goatsuckers, woodpeckers, ducks, and waders. Among reptiles, battles of
the males are known to occur in the cases of crocodiles, lizards, and
tortoises; among fishes, in those of salmon and sticklebats. Even among
insects the same law prevails; and male spiders, beetles of many groups,
crickets, and butterflies often fight together.
From this very general phenomenon there necessarily results a form of
natural selection which increases the vigour and fighting power of the
male animal, since, in every case, the weaker are either killed,
wounded, or driven away. This selection would be more powerful if males
were always in excess of females, but after much research Mr. Darwin
could not obtain any satisfactory evidence that this was the case. The
same effect, however, is produced in some cases by constitution or
habits; thus male insects usually emerge first from the pupa, and among
migrating birds the males arrive first both in this country and in North
America.
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