The diversity of biological systems, at different levels of the organisation of the material, forms one of the main characteristics of the organic universe -- the phenomenon of polymorphism. Numerous investigations have shown that polymorphism appears beginning from the intracellular organisation of the material up to the level of the biocenosis. This book is based on the notion of polymorphism as the co-existence in the dynamic balance of two or more morphophysiologically distinctive forms within the population.