In his famous experiments on pea plants, Gregor Mendel rejected this hereditary mechanism in favor of particulate inheritance by demonstrating that alternative versions of genes (alleles) account for variations in inherited characters, though he didn’t actually know about genes as such. Blending inheritance would quickly erode such variation, since all traits would be combined from one generation to the next until all individuals shared the same blended phenotype. This mode of inheritance, however, posed problems for Darwin’s theory of natural selection (1859), which depends on the existence of heritable trait variation in populations of organisms. Given observed patterns of resemblance between parents and offspring, blending inheritance may seem intuitively reasonable, as it did to many of Charles Darwin’s contemporaries. Under the now-discredited theory of blending inheritance, the hereditary material was conceived as a fluid that combines the traits from two individuals into phenotypically intermediate offspring.