COURTSHIP AND SEX 207 



" primary " differences refer to the reproductive organs, the 

 " secondary " to those in other parts of the body, such as 

 the larynx or the hair. It is clearer to follow Poll, Kammerer 

 and others in recognising (I.) essential orgonadial differences 

 which must be present if there are sexes at all — the differences 

 between ovaries and spermaries ; and (II.) accessory differ- 

 ences which may or may not be present, some of them 

 subsidiary to the reproductive organs, either internally or 

 externally, and others affecting extra-genital parts of the 

 body. The scheme of division, slightly modified from 

 Poll's and Kammerer 's, may be thus expressed : 



I. Essential or Gonadial In the reproductive organs proper. 



II. Accessory or Incidental : — 



(a) subsidiary to the gonads : — either internally, as in accessory 



glands ; or externally, as in pairing organs, egg-laying 

 organs. 



(b) somatic or extra-genital : — either internally, as in vocal 



organs ; or externally, as in colour, hair, feathers, etc. 



The sex differences have a structural and a functional 

 side, a morphological and a physiological aspect, but for 

 practical purposes one side may often be disregarded. Thus 

 a chitinous decoration on a male beetle has no vital activity 

 after it is formed ; it is the structural side that is important. 

 On the other hand, the differences in the blood of a male and 

 a female, which are of great importance, may not have any 

 detectable structural expression. Similarly, there are many 

 subtle differences in instincts and impulses, in physiological 

 habit and length of life, which are very real though we cannot 

 say much about their structural expression ! 



Another consideration to be kept in mind in the classifica- 

 tion of sex differences is the degree of permanence in their 

 expression. An adult peacock can never be confused with 

 a peahen, but there are many birds, e.g. some ducks, which 

 show great dissimilarity between the sexes at the breeding 

 season and great similarity at other times. In many fishes, 

 such as sticklebacks, the males are conspicuously different 

 from the females at the breeding time, but inconspicuously 

 different at other times. In short, there are many nuptial 

 characters which wax and wane according to the sexual 

 state of the organism. 



