A very extensive series of observations has been made on the character called abnormal abdomen. In pure cultures kept moist with abundance of fresh food all the flies that hatch for the first few days have the black bands of the abdomen obliterated or made faint and irregular. As the bottles get dry and the food becomes scarce the flies become more and more normal, until at last they are indistinguishable from the normal flies. Nevertheless these normal-looking flies will give rise in a suitable environment to the same kind of flies as the very abnormal flies first hatched. By breeding from the last flies of each culture, and in dry cultures, flies can be bred from normal ancestors for several generations, and then by making the conditions favorable for the appearance of the abnormal condition, the flies will be as abnormal as though their ancestors had always been abnormal. Here, then, is a character that is susceptible to the variations in the environment, yet whatever the realized condition of the soma may be, that condition has no effect whatever on the nature of the germ-plasm. A more striking disproof of the theory of the inheritance of acquired characters would be hard to find. A demonstration is given in this instance of the interaction between a given genotypic constitution and a special environment. The character abnormal is a sex-linked dominant. Therefore, if an abnormal male is mated to a wild female the daughters are heterozygous for abnormal, while the sons, getting their X chromosome from their mother, are entirely normal. In a wet environment all the daughters are abnormal and the sons normal. As the culture dries out the daughters' color becomes normal in appearance. But while the sons {17} will never transmit abnormality to any of their descendants in any environment, the daughters will transmit (if bred to normal males) in a suitable environment their peculiarity to half of their daughters and to half of their sons. The experiment shows convincingly that the abnormal abdomen appears in a special environment only in those flies that have a given genotypic constitution. As the cultures dry out the abnormal males are the first to change over to normal, then the heterozygous females, and lastly the homozygous females. It is doubtful if any far-reaching conclusion can be drawn from this series, because the first and second classes differ from each other not only in the presence of one or of two factors for abnormal, but also by the absence in the first case (male) of an entire X chromosome with its contained factors. The second and third classes differ from each other only by the abnormal factor.