240 EVOLUTION AND GENETICS 



care for the young brood and build comb for a longer period, and 

 finally fly out to gather nectar and pollen for food, and propolis 

 to stop up crevices in the hive. When there are no young workers 

 available, as in the spring, the older ones take up the work of the 

 hive in addition to their own duties afield. 



Communal life is nowhere more complex than in the human 

 species, but here it depends on differences of training, and not on 

 structural adaptation. This is made possible by the high develop- 

 ment of intelligence. 



Commensalism and Symbiosis. Different species are sometimes 

 associated for mutual benefit. If the association is not indis- 

 pensable, it is known as commensalism. A classic example is that 

 of the hermit crab and the hydroid which grows on its shell. The 

 crab is more or less protected by the disguise afforded by the 

 hydroid, and pro])ably by the latter's stinging cells, while the hy- 

 droid is carried from place to place and is able to benefit by the 

 scraps of food discarded by the crab (Fig. 146). If the association 

 is indispensable, it is known as symbiosis. A familiar example is 

 the association of fungi and algae to form lichens. Among animals 

 the white ants and their intestinal fauna of protozoa have recently 

 been proved equally necessary to each other. The termites eat 

 wood, but cannot digest the cellulose of which it is composed; 

 the protozoa digest the wood for the termites, and in turn receive 

 a favorable environment and plenty of food. 



Parasitism. Not all associations are mutually beneficial, how- 

 ever. Some organisms live upon or within others, and entirely 

 at their expense. Such organisms are called parasites, and those 

 upon which they live are known as hosts. Parasitism is extremely 

 varied and complex. It cannot be treated adequately in such an 

 account as this, but in all cases certain tendencies are to be noted. 

 In proportion to the dependence of the parasite, it degenerates 

 structurally and becomes incapable of independent life. Obligate 

 parasites are thus our most degenerate organisms. In proportion 

 to the risk involved in changing from host to host during repro- 

 duction or during the life of the individual, compensating adapta- 

 tions must be acquired. These are usually reproductive and are 

 either in the form of a very high rate of increase to meet enormous 

 destruction, or the elimination of the more vulnerable stages. 

 In proportion to the constancy of the association, the powers 

 of locomotion may be reduced. 



