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 which will stay and which 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 tile case of those animals which are (33) of doing each other great harm, there is a system for the (34) animal to show the winning animal that he wishes to (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) submission. 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. 32().

A. able
B. capable
C. fond
D. likely