202 GENERAL PRINCIPLES OF ZOOLOGY. 



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

 of the young offspring further leads to a closer organization, 

 to a state formation in the narrower sense ; all insect states 

 are built up on this basis. Consequently, since the sexual 

 life is the starting-point of state building, it is further con- 

 ceivable that, in the different groups of individuals ranking 

 as "states," the sexual organs are influenced in their 

 development. Besides males and females (kings and 

 queens) there are still other animals with degenerated sex- 

 ual organs 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 

 posterity, the duty of the workers is to care for the young 

 brood, to look after the hive, to provide food and protec- 

 tion, and also to serve for defence, if the latter is not dele- 

 gated 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 case is called parasitism, 

 the latter symbiosis. 



Parasitism. By parasites we understand animals which 

 find their dwelling-place upon and obtain nourishment from 

 another animal, the host, and which consequently have 

 come into a dependent condition and have undergone a 

 more or less extensive change in their organization. 



True Parasitism. The fact that an animal has set- 

 tled down upon another is not sufficient to characterize it 

 as a parasite. There are many animals which in general 

 are sedentary, and which, when opportunity offers, attach 

 themselves to a stone, a plant, or another animal ; in such 

 cases to speak of parasitism is a misnomer, because it can- 

 not be called a dependent condition. If a hydroid polyp 

 fastens itself upon the back of a crab instead of on a stone, 



