In principle, a new phenomenon might emerge without reflecting destructively upon any part of scientific practice. A new theory does not have to conflict with any of its predecessors. It might deal exclusively with phenomena not previously known. Or again, the new theory might be simply a higher-level theory than those known before; one that linked together a whole group of lower level theories can be conceived. Any and all of them might be exemplified by the historical process through which science has developed. If they were, scientific development would be genuinely cumulative. New sorts of phenomena would simply disclose order in an aspect of nature where none had been seen before. In the evolution of science new knowledge would replace ignorance rather than replace knowledge of another and incompatible sort.