Answer: (c). Allison found, as had E.A. Beet earlier, that sickle-cell disease and trait were in higher frequency in regions with the greatest number of cases of malaria. Knowledge of the etiology of the disease indicated how sickle-cell hemoglobin, in heterozygous individuals, could confer an advantage in combatting the spread of the malairal parasite in the body. The evidence was circumstantial, however, as the distributions represented only a correlation. Only much later did clinical laboratory studies confirm that in heterozygotes red blood cells containing malarial parasites more frequently sickled and where therefore removed from the blood stream more readily, preventing, spread of the disease.