资料题：What's the most important reason for people in the city to keep dogs now（　）。

Change-or the ability to adapt oneself to a changing environment-is essential to evolution.The farmer whose land is required for housing or industry must adapt himself: he can move to another place and master the problems peculiar to it; he can change his occupation, perhaps after a period of training; or he can starve to death.A nation which cannot adapt its trade or defense requirements to meet world conditions faces economic or military disaster.Nothing is fixed and permanently stable.There must be movement forward, which is progress of a sort, or movement backward, which is decay and deterioration.In this context, tradition can be a force for good or for evil.As long as it offers a guide, it helps the ignorant and the uninformed to take a step forward and, thereby, to adapt themselves to changed circumstances.Tradition, or custom, can guide the hunter as effectively as it can influence the nervous hostess.But if we make an idol of tradition, it ceases to become a guide and becomes an obstacle lying across the path of change and progress.if we insist on trying to plot the future by the pest, we clearly handicap ourselves and invite failure.The better course is to accept the help which tradition can give but, realizing that it necessarily has its roots in the pest,to be well aware of it limitations in changing world.