198 GENERAL PRINCIPLES OF ZOOLOGY. 



27. Many animals deposit their eggs before or shortly 

 after fertilization (oviparous) ; others lay eggs which have 

 already been fertilized in the maternal uterus, and at the 

 time of laying have passed through some of the stages of 

 development (ovo-viviparous). A third series of animals 

 give birth to living young (viviparous). 



28. The development of an animal is either direct or 

 indirect (metamorphosis). 



29. Indirect development or metamorphosis is where the 

 young animal, as it comes from the egg, differs from the 

 sexually mature animal in two points : 



(a) by the lack of certain organs which occur in the 



sexually mature animals ; 

 (&) by the appearance of organs, larval organs, which 



are lacking in the sexually mature animals. 



III. RELATION OF ANIMALS TO ONE ANOTHER. 



General Relations. Just as between the organs of one 

 and the same animal there exists a regular connection 

 which is termed correlation of parts, so also the different 

 individuals of the animal population stand in manifold 

 and intimate reciprocal relations to one another. Darwin 

 has shown in a great number of instances how the con- 

 ditions of existence of many animal species are altered, 

 if other forms appear anew or disappear, or an extraordi- 

 nary reduction or increase of the number of individuals 

 takes place. Such reciprocal actions are usually of a more 

 special nature and can be understood only by individual 

 study ; a few conditions are of wide occurrence and are 

 hence suitable for a general consideration ; to such belong 

 colony and state formation, parasitism, and symbiosis. 



I. RELATIONS BETWEEN INDIVIDUALS OF THE 

 SAME SPECIES. 



Colony Formation. Colony and state formation are 

 conditions which exist between individuals of the same 

 species. By an animal colony is understood a union of 



