Since early times interest has centred round the problem of whether special virtue or taint is attached to the first-born child. Some recent workers, like Pearson (1914), have believed the first-born child to be liable to physical and mental handicaps. Still (1927) focuses attention on certain diseases which appeared to be more frequently manifested in the first child in a family than in children born later in that family. He also realized that maternal age might be a significant factor in other conditions. A summary of a good deal of the work done on these problems was published by Thurstone and Jenkins (1931). These writers drew attention also to parallel studies in animal genetics. Wright (1926 and 1936), by his analysis of Polydactyly and coat colour in guinea-pigs, laid a foundation for genetical work on maternal age. The effects he observed were due to alterations of characters in early offspring. Unfortunately, experimental animals rarely live long enough to make possible the corresponding study of maternal age effects at the end of the reproductive period. Evidence that birth order of itself is of significance in animal genetics has been scanty. Recently, however, Goetsch (1937) reported that the first eggs of the queen ant (Pheidole pallidula) give rise to small and physically feeble workers. In the present communication the results of some of the writer's recent researches on the effects of maternal age and birth order in human genetics are discussed. A multitude of human malformations are relevant to this study. Investigations of their respective ætiologies is of interest from the point of view of elucidating the effects of natural selection in man as well as from the standpoint of preventive medicine.