NATURAL SELECTION AND THE
MENTAL CAPACITIES OF MANKIND
The fundamental mechanisms of the transmission of heredity from parents to offspring are surprisingly uniform in most diverse organisms. Their uniformity is perhaps the most remarkable fact disclosed by genetics. The laws discovered by Mendel apply to human genes just as much as to those of the maize plant, and the processes of cellular division and germ cell maturation in man are not very different from those in a grasshopper. The similarity of the mechanisms of heredity on the individual level is reflected on the population level in a similarity of the basic causative factors of organic evolution throughout the living world. Mutation, selection, and genetic drift are important in the evolution of man as well as in amoebae and in bacteria. Wherever sexuality and cross-fertilization are established as exclusive or predominant methods of reproduction, the field of hereditary variability increases enormously as compared with asexual or self-fertilizing organisms. Isolating mechanisms which prevent interbreeding and fusion of species of mammals are operative also among insects.
Nevertheless, the universality of basic genetic mechanisms and of evolutionary agents permits a variety of evolutionary patterns to exist not only in different lines of descent but even at different____________________