54 COMPAEATIVE ANATOMY. 



and the same gland produces "botli semen and ova (hermaplirodite 

 gland). The ducts, also, are often more or less common to them 

 both. But in other forms the genital organ is divided, the products 

 of its two parts being different ; testes and ovaries, that is, are 

 present as separate organs, the excretory organs of which only are 

 united more or less extensively; or each of them may have its 

 separate orifice. All those animals which unite in themselves both 

 kinds of reproductive organs are known as Hermaphrodites. 

 A separation of sexes is apparently foreshadowed in various forms, 

 by the alternating activity of the organs, at one time the egg- 

 forming and at another time the sperm-forming organ exercising its 

 function. 



The hermaphrodite stage is the lower, and the condition of dis- 

 tinct sexes has been derived from it. This change is due to the 

 decrease in size of one or the other organ, so that hermaphroditism 

 is the precursor of sexual differentiation. This differentiation, by 

 the reduction of one kind of sexual apparatus, takes place at very 

 different stages in the development of the organism, and often when 

 the sexual organs have attained a very high degree of differentiation. 

 In these cases ontogeny exhibits the two kinds of organs primitively 

 united, and so causes the individual to be hermaphrodite at a certain 

 stage in development. 



The separation of the sexes affects the whole of the organism, 

 for it produces a series of changes in each sex, which affect organs 

 that had primitively little to do with the sexual function. Sexual 

 differentiation is completed when the two kinds of organs are 

 given over to different individuals. Thenceforward for reproduction, 

 not only two different substances, semen and ova, and two different 

 organs for producing them, are necessary, but also two individuals ; 

 these are distinguished as male and female. 



Changes in the Organs. 



Development and Degeneration. 



§47. 



The result of the continued differentiation of a given organ is a 

 complication by which the organ is removed proportionately further 

 from its primitive condition. As the primitive condition is the lower 

 differentiation, it entails a perfecting corresponding to a higher con- 

 dition. This is clear on the principle of division of labour, which is 

 the cause of all differentiation (cf. § 12). In obedience to this law 

 a function can be the more perfectly carried out, the more exclu- 

 sively the organ is related to that function. The more an organ is 

 exercised for one function only, the more suitable are the conditions 

 for its development in one direction, for there is no competition with 



