l.-iij GENERAL rRINCIELES OE ZOOLOGY 



sisters; its ^■cry existence therefore has lieconie dependent upon these; 

 the single indi\'idual can li\'e onlv wliile a part of a whole. Thus divi- 

 sion of hxbor leads to greater centralization; the more polymorpliic 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 im])ortance in colonies, so here 

 tlie sexual plays a pirominent role. Untler the inlluence of the sexual 

 impulse, many animals, even some of the lowest organisms, llock together, 

 either permanently or iieriodieallv; sea-urchins, sea-cueuml>ers, manv 

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

 herds of deer, elephants, etc. The care of the voung further leads to a 

 closer organization, to a society. All insect societies are liuilt up on this 

 basis. Consequently, since the sexual life is the starting-point of social 

 life, in the different groups of individuals forming the communitv, the 

 sexual organs mav lie intluenced in their de\'eloimient. Besides males 

 and females (lungs 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). \Miile 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). 



IL Rel.-vtions BETWf.i'.N Individu.als or DiFFicRENT Snccii-s. 



Where individuals of different species stand in close reciprocal rela- 

 tions to each other the cause is in the advantages which the one deri\es 

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

 is called parasilisDi, the latter syniljiosis. 



Parasitism. — Parasites are organisms which dwell upon or in 

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

 ha\-e conseipiently come into a dependent condition and have undergone 

 a more or less extensive change in tlieir organization. 



Degeneration Caused by Parasitism. — The degree to wliich a 

 parasite has become deijcndent 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 



