260 
THE PKINCIPLES OE 
Part M. 
never known the females of any species to arrive before 
their males. During one spring he shot thirty-nine 
males of Eay’s wagtail (. Budytes Rail) before he saw a 
single female. Mr. Gould has ascertained by dissection^ 
as he informs me, that male snipes arrive in this 
country before the females. In the case of fish, at the 
period when the salmon ascend our rivers, the males in 
large numbers are ready to breed before the females. 
So it apparently is with frogs and toads. Throughout 
the great class of insects the males almost always 
emerge from the pupal state before the other sex, so 
that they generally swarm for a time before any females 
can be seen . 3 The cause of this difference between 
the males and females in their periods of arrival ancl 
maturity is sufficiently obvious. Those males which 
annually first migrated into any country, or which in 
the spring were first ready to breed, or were the most 
eager, would leave the largest number of offspring ; 
and these would tend to inherit similar instincts and 
constitutions. On the whole there can be no doubt that 
with almost all animals, in which the sexes are separate,, 
there is a constantly recurrent struggle between the 
males for the possession of the females. 
Our difficulty in regard to sexual selection lies in 
understanding how it is that the males which conquer 
other males, or those which prove the most attractive 
to the females, leave a greater number of offspring 
to inherit their superiority than the beaten and less 
3 Even with those of plants in which the sexes are separate, the male 
flowers are generally mature before the female. Many hermaphrodite' 
plants are, as first shewn by 0. K. Sprengel, dichogamous ,* that is, 
their male and female organs are not ready at the same time, so that 
they cannot be self-fertilised. Now with such plants the pollen is 
generally mature in the same flower before the stigma, though there 
are some exceptional species in which the female organs are mature- 
before the male. 
