SEXUAL SELECTION 281 



of tlie United States. ' The majority of the maie salmon in 

 our rivers, on coming up from the sea, are ready to breed 

 before the females. So it appears to be with frogs and 

 toads. Throughout the great class of insects the males 

 almost always are the first to emerge from the pupal state, 

 so that they generally abound for a time before any females 

 can be seen.' The cause of this difference between the 

 males and females in their periods of arrival and 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. It must be borne in 

 mind that it would have been impossible to change very 

 materially the time of sexual maturity in the females, with- 

 out at the same time interfering with the period of the pro- 

 duction of the young — a period which must be determined 

 by the seasons of the year. 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 under- 

 standing 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 superi- 

 ority than their beaten and less attractive rivals. Unless 

 this result does follow, the characters which give to certain 

 males an advantage over others could not be perfected and 

 augmented through sexual selection. When the sexes exist 

 in exactly equal numbers, the worst-endowed males will 

 (except where polygamy prevails) ultimately find females, 



5 J. A. Allen, on the "Mammals and "Winter Birds of Florida," BuU. Oomp. 

 Zoology, Harvard OoUege, p. 268. 



' Even -with those plants in which the sexes are separate, the male flowers 

 are generally mature before the female. As first shown by 0. K. Sprengel, 

 many hermaphrodite plants are dichogamous ; that is, thejr male and female 

 organs are not ready at the same time, so that they cannot be self-fertilized. 

 Now in such flowers the pollen is in general matured before the stigma, though 

 there are exceptional cases in which the female organs are beforehand. 



