Is the magnetic field in the Earth's magnetotail static? if yes, why? if not, what causes the magnetic field to change and how does it evolve with time? These questions have haunted magnetospheric physicists for the past decade. Although significant progress has been made in this area of research, a consensus still does not exist. This thesis approaches the problem from the most fundamental basis--Faraday's law relating the curl of the electric field to the time variation of the magnetic field. If we can reach an independent theory that relates the electric field to the magnetic field, the whole problem can, at least in principle, be solved. This thesis pursues the problem both physically and mathematically. Our answers to the questions listed at the beginning are: (1) the magnetic field is generally not static; (2) the change is powered by the energy transfer from the solar wind to the magnetosphere, the agent that effects the change is plasma injection from the high-latitude plasma mantle; (3) the time-dependence is closely related to the velocity distribution of the mantle plasma; A decrease of B\sbz in the near tail and a flux buildup at the farther end of tail are two primary features of the time evolution; (4) a dense, drifting plasma mantle causes an intensive reconfiguration in the plasma sheet and is likely to lead to plasma sheet instability. The general results of the thesis are supportive of Hones' phenomenological model of the tail evolution (Hones, 1977).