GENERAL SUMMART. 609 



Is intelligible if we admit the action of one common cause, namely 

 sexual selection. 



Sexual selection depends on the success of certain individuals 

 over others of the same sex, in relation to the propagation of the 

 species; whilst natural selection depends on the success of both 

 sexes at all ages, in relation to the general conditions of lite. 

 The sexual struggle is of two kinds; in the one it is between the 

 individuals of the same sex, generally the males, in order to drive 

 away or kill their rivals, the females remaining passive; whilst 

 in the other, the struggle is' likewise between the individuals of 

 the same sex, in order to excite or charm those of the opposite 

 sex, generally the females, which no longer remain passive, but 

 select the more agreeable partners. This latter kind of selection 

 is closely analogous to that which man unintentionally, yet ef- 

 fectually, brings to bear on his domesticated productions, when 

 he preserves during a long period the most pleasing or useful 

 individuals, without any wish to modify the breed. 



The laws of inheritance determine whether characters gained 

 through sexual selection by either sex shall be transmitted to the 

 same sex, or to both; as well as the age at which they shall be 

 developed. It appears that variations arising late in life are 

 commonly transmitted to one and the same sex. Variability is 

 the necessary basis for the action of selection, and is wholly 

 independent of it. It follows from this, that variations of the 

 same general nature have often been taken advantage of and 

 accumulated through sexual selection in relation to the propaga- 

 tion of the species, as well as through natural selection in relation 

 to the general purposes of life. Hence secondary sexual char- 

 acters, when equally transmitted to both sexes can be distin- 

 guished from ordinary specific characters only by the light of 

 analogy. The modifications acquired through sexual selection are 

 often so strongly pronounced that the two sexes have frequently 

 been ranked as distinct species, or even as distinct genera. Such 

 strongly-marked differences must be in some manner highly im- 

 portant; and we know" that they have been acquired in some in- 

 stances at the cost not only of inconvenience, but of exposure 

 to actual danger. 



The belief in the power of sexual selection rests chiefly on 

 the following considerations. Certain characters are confined to 

 one sex; and this alone renders it probable that in most cases 

 they are connected with the act of reproduction. In innumer- 

 able instances these characters are fully developed only at ma- 

 turity, and often during only a part of the year, which is always 

 the breeding-season. The males (passing over a few exceptional 

 cases) are the more active in courtship; they are the better 

 armed, and are rendered the more attractive in various ways. 

 It is to be especially observed that the males display their at- 

 tractions with elaborate care in the presence ol the females; and 



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