[Theoretical approaches to the description of the morphology of organisms].
Theoretical morphology should include methods that allow to determine limits of discrete variability of organisms of the given taxon. It is possible to determine the multitude of possible factors based on the demands of natural selection to the given structure under given conditions. The author presents a number of such multitudes that describe the variability of salamander tongue apparatus, kinematics of flapping flight in insects and wing shape of birds from the order Ciconiiformes. Another approach is based on mechanisms that rule the morphogenesis of individuals of the given taxon in particulate environment. Examples of morphogenetic multitude are illustrated by the variability of location of organs on plant stems, morphology of mollusc shells and colonies of hydroids polyps. All examples show that existing phenotypes occupy a compact area in the space of theoretically possible forms so the multitude of realized phenotypes does not have free space. If some free space is discovered it means that corresponding forms of life exist in nature but still are not registered and investigated.