212 The Descent of Man. Part IJ, 



whicli sexual selection aots is somewhat uncertain. Neverthe- 

 less if those naturalists who already believe in the mutability of 

 apeoies, will read the following chapters, they will, I think, agree 

 with me, that sexual selection has played an important part in 

 the history of the organic world. It is certain that amongst 

 almost all animals there is a struggle between the males for the 

 possession of the female. This fact is so notorious that it would 

 be superfluous to give instances. Hence the females have the 

 opportunity of selecting one out of several males, on the suppo- 

 sition that their mental capacity suffices for the exertion of a 

 choice. In many cases special circumstances tend to make the 

 struggle between the males particularly severe. Thus the males 

 of our migratory bijrds generally arrive at their places of breeding 

 before the females, so that many males are ready to contend for 

 each female. I am informed by Mr. Jenner Weir, that the bird- 

 catchers assert that this is invariably the case with the nightin- 

 gale and blackcap, and with respect to the latter he can himself 

 confirm the statement. 



Mr. Swaysland of Brighton has been in the habit, during the 

 last forty years, of catching our migratory birds on their first 

 arrival, and he has never known the females of any species to 

 arrive before their males. During one spring he shot thirty -nine 

 males of Eay's wagtail {Bwlytes JRai.i) before he saw a single 

 female. Mr. Gould has ascertained by the dissection of those 

 snipes which arrive the first in this country, that the males come 

 before the females. And the like holds good with most of the 

 migratory birds of the United States.^ The majority of the male 

 salmon in our rivers, on coming up from tho 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 niamber of offspring ; and these 



' J. A. Allon, on the ' Mammals rndite plants are dichogamous ; that 



and Winter Birds of Florida,' Bull, is, their male and female organs are 



Comp. Zoology, Harvard College, p. not ready at the same time, so that 



268. they cannot be .self-fertilised. Now 



' Even with those plants in which in such flowers, the pollen is in 



the sexes are separate, the male general matured before the stigma, 



flowers are generally mature be- though there are exceptional cases 



fore the female. As first shewn in which the female organs an 



hv 0. K. Spreug*'!, anany heiinaph- beforehand. 



