Without death there is little innovation. Extinction - death of a species - is part and parcel of evolutionary change. In the absence of this kind of extinction new developments would not prosper. In our own history, periods when ideas have been perpetuated by dogma, preventing the replacement of old by new ideas, have also been times of stultifying stagnation. The Dark Ages in western society were the most static, least innovative of times. So the fact that trilobites were replaced by batches of successive species through their long history was a testimony to their evolutionary vigour.