Using complex mathematics, Motoo Kimura calculated that genomes constantly undergo a remarkably high number of mutations per unit of time. He wrote, "Calculating the rate of evolution in terms of nucleotide substitutions seems to give a value so high that many of the mutations involved must be neutral ones."  Motoo Kimura's theory holds that most evolutionary changes occur at the molecular level, and that most of the variation within and between species is due to random genetic drift of mutant alleles that are selectively neutral -- neither advantageous or disadvantageous. Kimura does not discuss natural selection in his paper; however, his theory does not contradict traditional Darwinian theory that evolution occurs through the natural selection of non-neutral, advantageous variants in a given population. Kimura expanded his theory in his 1983 book The neutral theory of molecular evolution.

In 2021 a digital facsimile of the 1968 paper was available from blackwellpublishing.com at this link.

(Thanks to Juan Weiss for this reference and its interpretation.)