A major reason for 【21】______ in the animal world is territory. The male animal establishes an area. The size of the area is sufficient to provide food for him, his 【22】______ and their offspring. Migrating birds, for example, 【23】______ up the best territory in the order of 'first come, first 【24】______ . 'The late arrivals may acquire larger territories, but less food is 【25】______ , or they are too close to the 【26】______ of the enemies of the species. 【27】______ there is really insufficient food or the danger is very great, the animal will not mate. In this way, the members of the species which are less fit will not have 【28】______ . When there is conflict 【29】______ territory, animals will commonly use force, or 【30】______ of force, to decide who will stay and who will go. It is interesting to note, however, that animals seem to use the 【31】______ amount of force necessary to drive away the 【32】______ There is usually no killing. In the case of those animals which are 【33】______ of doing each other great harm, (here is a system for the 【34】______ animal to show the winning animal that he wishes 【35】______ When he shows this, the 【36】______ normally stops fighting. Animals especially birds, which can easily escape from conflict seem to have no obstacle against killing, equally no mechanism 【37】______ sub mission. The defeated bird simply flies away. However, if two doves are 【38】______ in a cage, and they start fighting, they will continue to fight until one kills the other. We all think of the dove as a 【39】______ of peace and, in its natural habitat, it is peaceful. But the' peace' mechanism does not 【40】______ in a cage. 【21】