156 GENERAL PRINCIPLES OF ZOOLOGY 



sisters; its very existence therefore has become dependent upon these; 

 the single individual can live only while a part of a whole. Thus divi- 

 sion of labor leads to greater centralization ; the more polymorphic a 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 there is no organic connection, only a voluntary com- 

 munal life. As asexual reproduction is of importance in 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 permanently or periodically; sea-urchins, sea-cucumbers, many 

 fishes, collect near the coast at the time of egg-laying; it draws together 

 herds of deer, elephants, etc. The care of the young 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, 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 other animals with degenerated 

 sexual organs incapable of function, the workers; the latter are either 

 only rudimentary females (bees and ants) or rudimentary 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, pro- 

 tection, and defence, if the latter be not delegated to a special class, the 

 soldiers (termites). 



II. RELATIONS BETWEEN INDIVIDUALS OF DIFFERENT SPECIES. 



Where individuals of different species stand in close reciprocal rela- 

 tions to each other the cause is in the advantages which the one derives 

 from the other, or which both furnish reciprocally; the former condition 

 is called parasitism, the latter symbiosis. 



Parasitism. Parasites are organisms which dwell upon or in 

 another organism, the host, and obtain nourishment from it. They 

 have consequently come into a dependent condition and have undergone 

 a more or less extensive change in their organization. 



Degeneration Caused by Parasitism. The degree to which a 

 parasite has become dependent upon its host is determined by the extent 

 to which the parasite has adapted itself to the organization of its host. 

 Therefore it is necessary, in speaking of parasitism, to consider the modi- 

 fications which the parasitic life has caused in the structure. These 

 concern most immediately the organs of locomotion and nutrition. Since 



