(ECOLOGY. 167 



to its brothers and sisters; its very existence therefore has become 

 dependent upon these; the single individual can live only while a 

 part of a whole. Thus also division of labor leads to greater 

 centralization; the more polymorphic an animal colony becomes, 

 the more unified it is, the more it gives the impression of being a 

 single animal instead of an aggregation of single animals. 



In Social Animals the reciprocal dependence of the individuals 

 is much less, since here there exists no organic connexion, only a 

 voluntary communal life. As asexual reproduction is of impor- 

 tance in the case of colonies, so here the sexual plays a prominent 

 role. Under the influence of the sexual impulse, many animals, 

 even some of the lowest organisms, flock together, either per- 

 manently or periodically; sea-urchins, sea-cucumbers, many fishes, 

 collect near the coast at the time of egg-laying. The sexual im- 

 pulse draws together herds of deer, elephants, etc. The care of the 

 young offspring further leads to a closer organization, to a society. 

 All insect societies are built up on this basis. Consequently, 

 since the sexual life is the starting-point of social life, it is further 

 comprehensible that, in the different groups of individuals forming 

 the community, the sexual organs may be influenced in their 

 development. Besides males and females (kings and queens) 

 there are still other animals with degenerated sexual organ? 

 incapable of function, the workers; the latter are either only 

 females (bees and ants) or females and males (termites). While 

 the kings and queens give rise to the next generation, the workers 

 care for the young, look after the hive, provide food and protec- 

 tion, and also serve for defence, if the latter is not delegated to a 

 special class, the soldiers (termites). 



II. RELATIONS BETWEEN INDIVIDUALS OF DIFFERENT SPECIES. 



Causes of Close Relation. Where individuals of different 

 species stand in close reciprocal relations to each other the cause 

 is to be found in the advantages which the one species derives from 

 the other, or which these both furnish reciprocally; the former 

 condition is called parasitism, the latter symbiosis. 



Parasitism. Parasites are animals which find their dwelling- 

 place upon or in another animal, the host, and obtain nourishment 

 from it. They have consequently come into a dependent condi- 

 tion and have undergone a more or less extensive change in their 

 organization. 



True Parasitism. The fact that an animal has settled down upon 

 another is not sufficient to characterize it as a parasite. There are many 



