Chap. XXI. General Summary. 615 



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

 developed. It appears that variations arising late in life are 

 commonly transmitted to one and the same sex. Variability ia 

 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. Henee secondary 

 sexual characters, when equally transmitted to both sexes can be 

 distinguished 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 important ; and wo know that they have been 

 acquired in some instances 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 

 innumerable instances these characters are fully developed only 

 at maturity, 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 

 attractions with elaborate care in the presence of the females ; 

 and that they rarely or never display them excepting during 

 the season of love. It is incredible that all this should be 

 purposeless. Lastly we have distinct evidence with some quad- 

 mpeds and birds, that the individuals of one sex are capable of 

 feeling a strong antipathy or preference for certain individuals 

 of the other sex. 



Bearing in mind these facts, and the marked results of man's 

 unconscious selection, when applied to domesticated animals and 

 cultivated plants, it seems to me almost certain that if the 

 individuals of one sex were during a long series of generations to 

 prefer pairing with certain individuals of the other sex, charac- 

 terised in some peculiar manner, the offspring would slowly but 

 surely become modified in this same manner. I have not 

 attempted to conceal that, excepting when the males are more 

 numerous than the females, or when polygamy prevails, it is 

 'loubtfuJ how the more attractive males succeed in leaving a 



