I5 GENERAL PRINCIPLES OF ZOOLOGY. 



12. The male and female sexual organs serve for re- 

 production. 



13. Either of the two kinds of sexual organs may be 

 present in an animal (gonochorism), or both may be found in 

 one and the same animal (hermaphroditism). 



14. The highest degree of hermaphroditism is attained 

 when one and the same gland (the hermaphroditic gland) 

 gives rise to both eggs and spermatozoa. 



15. Very often the sexual organs and the ducts from 

 the kidneys are closely united ; we then speak of a 

 uro-genital system. 



IV. PROMORPHOLOGY, OR STUDY OF THE FUNDA- 

 MENTAL FORMS OF ANIMALS. 



Organic and Inorganic Bodies. The structure of the 

 individual animal rests upon the regular combination of 

 differently-functioning organs. The organs thereby assume 

 a relation to one another which is definite for each animal 

 group, or varies only in subordinate ways. If the various 

 animal groups are compared with reference to the principle 

 of the arrangement of parts, there appear some few funda- 

 mental forms which play in morphology a role similar to 

 that of the fundamental forms of crystals in mineralogy. 

 But we must not, by pushing this comparison too far, at- 

 tempt to compare the study of the fundamental forms, the 

 promorpJiology, of animals with crystallography as of equal 

 value. A crystal is a mass made up of similar parts; its- 

 form is the necessary and immediate result of the chemico- 

 physical constitution of its molecules. A direct connection 

 of this kind between molecular structure and fundamental 

 form does not, and cannot, exist in the organism, since each 

 organ is composed of manifold chemical combinations. 

 Consequently there is lacking also the mathematical regu- 

 larity which occurs in crystals. Even in the case of animals 

 which have the greatest regularity in the arrangement of 

 their parts, there is not an entire conformity to the de- 

 mands of the fundamental form, so that we are compelled 



