 It is not strictly correct to assert that Mendelian experiments are unable to examine species-typical traits. For example, all individuals of all species of the fruit-fly genus Drosophila have exactly three simple light receptors, ocelli, arranged in a symmetrical triangle on the midline of the top of their heads. The simplest assumption is that there is no variation in genotypes (in the sense of material constituents) that influence this trait and its development is resistant to normal environmental disturbance. However, if the development of the fly is sufficiently disturbed, some flies with two or fewer ocelli are observed. If those with fewer than three ocelli are used as parents for the next generation, they produced more abnormal flies than the parental generation. When the process of selective breeding from abnormal flies is continued over many generations, a line of flies is produced that consistently has two ocelli, even in the absence of any external disturbance of development (Maynard Smith &amp; Sondhi 1960). The success of this and other selection experiments in the same vein shows that the original uniformity of the trait is not an indication that there is uniformity in all of the genotypes that may, under certain conditions, influence the trait’s development (Rendel 1967). Any investigation of how the diverse genotypes result in the development of the typical three-ocelli pattern now has to explain the occurrence of the aberrant pattern as well.