At certain times in the past, geomagnetic polarity reversals have caused fluctuations in the strength of the magnetospheric field. These in turn have modulated the cosmic ray flux impinging on the earth's surface and induced corresponding changes in the sea level radiation environment. The magnitude of the increase in the over-all background dose rate during geomagnetic field strength minima is a factor of 2 over the present dose rate. The maximum decrease during geomagnetic field strength maxima is undefined. The correlation in time between geomagnetic polarity reversals and evolutionary accelerations indicates that the increased intensity of ionizing radiation at those times may have enhanced the general mutation rate of organisms.