PRESIDENTIAL ADDRESS. 21 



Sexual selection is the law that one sex, usually the 

 female, exercises a choice between the individuals of the 

 other, whereb}' those individuals possessing the selected qual- 

 ities stand a much greater chance to have the opportunit3^ of 

 transmitting them to their offspring. This law explains the 

 greater ornamentation of the males in so many species, since 

 most such characters are peculiar to one sex and only appear 

 at maturity. Sexual selection also checks the tendency of 

 natural selection to extreme variation in certain directions, 

 since the sexes are well known to prefer their opposites, 

 which causes the offspring to occup)- a mean between the 

 extremes. This effect is very marked in the human race, but 

 is doubtless operative among the lower animals. As I pointed 

 out in last year's address, sexual selection has wrought a 

 great revolution in the relative size, strength, and beauty of 

 the two sexes, and reversed in birds and mammals the normal 

 law of female superiority which prevails in most of the lower 

 departments of life. 



ACQUIRED CHARACTERS. 



It will be readily perceived from what has been said of the 

 two great principles of transformism, the functional, as set 

 forth by Lamarck, and the selective, as elaborated by Darwin, 

 that the fundamental distinction between them is that in the 

 former the transforming qualities which are to be cumulativelj^ 

 transmitted through heredity to the descendants of a given 

 ancestral pair are acquired during the lifetime of these indi- 

 viduals, whereas in the latter the transforming increment is a 

 merely accidental modification arising from unknown causes 

 and hence called spontaneous. The theorj^ is that such .spon- 

 taneous variations are constantly taking place in all individ- 



