32 HEREDITY AND SEX 



relation to each other, forms one of the most curious 

 chapters in the evolution of sex, for it involves court- 

 ship between the males and females; the pairing or 

 union of the sexes and subsequently the building of 

 the nest, the care, the protection and feeding of the 

 young, by one or both parents. The origin of these 

 types of behavior is part of the process of evolution of 

 sex ; the manner of their transmission in heredity and 

 their segregation according to sex is one of the most 

 difficult questions in heredity one about which noth- 

 ing was known until within recent years, when a 

 beginning at least has been made. 



A few samples taken almost at random will illustrate 

 some of the familiar features in the psychology of sex. 

 Birds have evolved some of the most complicated types 

 of courtship that are known. It is in this group, too, 

 as we have seen, that the development of secondary 

 sexual characters has reached perhaps its highest types. 

 But it is not necessarily in the species that have the 

 most striking differences between the sexes that the 

 courtship is most elaborate. In pigeons and their 

 allies, for example, the courtship is prolonged and elab- 

 orate, yet the males and females are externally al- 

 most indistinguishable ; while in the barnyard fowl and 

 in ducks the process is relatively simple, yet chanti- 

 cleer is notoriously overdressed. 



Even in forms so simply organized as the fishes it is 

 known that the sexual instincts are well developed. 

 In the common minnow, fundulus, the males develop in 

 the breeding season elaborate systems of tactile organs. 

 The male swims by the side of the female, pressing 

 his body against her side, which causes her to set free 



