394 EVOLUTION AND GENETICS 



proboscis was of the same length. Among the millions of flowers 

 of the Angraecum produced every year, some would always be 

 shorter than the average, some longer. The former, owing to 

 the structure of the flower, would not get fertilized, because the 

 moths could get all the nectar without forcing their trunks down 

 to the very base. The latter would be well fertilized, and the 

 longest would on the average be the best fertilized of all. By this 

 process alone the average length of the nectary would annually 

 increase, because, the short-nectaried flowers being sterile and the 

 long ones having abundant offspring, exactly the same effect 

 would be produced as if a gardener destroyed the short ones and 

 sowed the seed of the long ones only; and this we know by experi- 

 ence would produce a regular increase of length, since it is this 

 very process which has increased the size and changed the form 

 of our cultivated fruits and flowers." Wallace carries the example 

 on to greater lengths, but this alone is a sufficient illustration of 

 the logic of the theory. 



Sexual Selection. As a corollary to his theory of natural 

 selection Darwin proposed the theory of sexual selection. This is 

 based upon the same fundamental principles as his chief theory, 

 but takes note of the choice exercised by the female in her selection 

 of a mate as the factor which determines the variation to be 

 preserved. 



It is well known that animals at the mating season display 

 their qualities to the best advantage and parade them before the 

 opposite sex. Ordinarily the male alone finds it necessary 

 to woo a mate by these means, but Beebe's observations of 

 the tinamou, already mentioned, show the possibility of a complete 

 reversal of these instincts. Even common birds are well known 

 for their display of color and song during the mating season, and 

 a few species, such as the ruby-throat humming bird and the 

 yellow-breasted chat, go through wonderful evolutions in the air 

 which show remarkable powers of flight. Obviously the female 

 makes a choice from among her available suitors. Granting that 

 there is a definite basis for this choice, and that the things attrac- 

 tive to one female are those normally attractive to others of the 

 species, we may readily see that the variations in the selected 

 males would be preserved in their offspring. The characters of 

 rejected males would die out for lack of an equal opportunity to 

 be perpetuated. 



