I have found this matter for the first time that each animal or plant that has a stronger blood/ fluid circulatory system could have a larger bulk. Today's insects can never grow to the size of larger animals because they lack the pipes and pumping system necessary to counteract the influence of gravity. In my theory embraces the important relationship between structure, gravity and function. The size of an organism is contingent upon the type of its circulatory system. One can think of this as a scale beginning with the smallest animals and ending with the largest. The smallest of animals lack a circulatory system; the slightly larger ones have an open circulatory system. Increasing in size, organisms such as crustaceans (crabs) and some bivalve mollusks (clams) have semi-open circulatory system which comprise some veins and arteries, without capillaries. This followed by still larger animals that have a closed circulatory system. Within the group that has a closed circulatory system are found different types of hearts. mammals are the largest animals because they have the most developed circulatory system with 4-chambered heart.