484 LIFE : OUTLINES OF GENERAL BIOLOGY 



Sex Differences. 



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 ; 

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

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

 Both kinds of accessory sex differences may be con- 

 trolled in their expression by special hormones liberated 

 from the gonads. 



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Fio. 6(). 



Sex Dimorphism in Birds of Paradise. The lower bird is the decorative male 

 and the upper the plain female of Paradisea minor. 



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. In a few insects 

 there is actually a difference in the colour of the blood. Similarly, 

 there are many subtle differences in instincts and impulses, in 



