Distinguished for his contributions to cytology, particularly of insects. He is especially known for his major contributions to the population genetics of grasshoppers and as the leading world authority on animal cytology. He is the author of an important monograph on animal cytology and evolution. He has made substantial contributions to a diversity of subjects, such as cytotaxonomy, hybridity, and the evolution of sex chromosomes. He made pioneer observations of the effects of X-rays on chromosomes and of the influence of temperature on chiasma frequency.