At the present time, mutation processes are considered to be a natural and essential part of Darwinistic evolution. However, at the beginning of the 20th century, this point of view was far from being a matter of fact and there was even a separate evolutionary theory, termed mutationism, which was considered by its proponents to be an alternative evolutionary theory that was incompatible with Darwinism. For example, when an orthodox mutationist explained the formation of wings in the ancestor of a certain clade of winged insect, he based his arguments on the concept that the members of the relevant clade of the wingless species produced winged mutants more rapidly than these mutants produced wingless individuals – revertants. Thus, winged individuals occurred in nature with increasing frequency until they completely predominated in the given clade.
Mutationists diminished the importance of natural selection and were willing to consider it to be, a most, a factor that removes unsuitable mutations. They overlooked the fact that, in the absence of this factor, they are not capable of explaining the most interesting phenomenon in biological evolution, i.e. the formation of adaptive traits, complicated yet useful structures and patterns of behaviour.