In the tropical rain forest many entire plant families and many very large genera are frequently confined within the limits of such a community, e.g. Dipterocarpaceae in the tropics of Asia. It has been established by many investigators that most plant species (especially trees) of the tropical rain forest are characterised by very small numbers of individuals, being represented by populations with very low densities (one, two or three individuals per hectare). Occasional instances of the dominance of a certain species in the upper storey (Stratum A) do not break the general principle, viz. the existence of a very large number of tree species always represented by small populations in any part of the tropical rain forest. Small size of populations of tree species, low population density and partial, though considerable, biotic isolation between populations and even between separate individuals, associated with the absence of seasonal rhythm and extreme irregularity of flowering and the consequent difficulty of cross-pollination - all these factors create favorable conditions for the process of speciation in which the role of genetic drift prevails over that of natural selection. Under such conditions mutant genes accumulate in populations and contribute to the relatively rapid origin of series of closely allied species, differing considerably from one another morphologically but possessing many "indifferent" characters. Theoretically it might be assumed that the origin of a series of closely allied species was the result of only a few mutations causing changes directly in a relatively small number of characters, but leading indirectly, through the consequent changes in morphogenesis involving different plant organs, to changes in many other characters.