Stresses and procedures to overcome stresses react with each other in various ways. Some kinds of interactions are sequential additivity, various types of synergism, and various types of antagonism which at times are protective. The kind of interaction is related to whether or not the stresses or limiting factors represent Liebig or Mitscherlich conditions. Crop yields are related to all these phenomena. Because of many different situations in which sequential additivity explains the interactions among two or more stresses (or promoters) acting simultaneously on biological systems, it is suggested that sequential additivity can be one of the unifying concepts in biology. Sequential additivity is that if two or more single stresses result in various reductions in activity, the result when they stress simultaneously is the product of the numerical values of the degree of stress of each relative to 1.0. Reverse but identical effects hold for promotors. Observed or reported examples include multiple trace element toxicities in plants, effects of multiple management practices in improving crop yields in agriculture, combined effects of tobacco and alcohol on human health, effects of two simultaneously applied herbicides on plants and effects of two trace elements not known to be essential to plants on plant growth when applied simultaneously.