Though often puny at birth, twins and triplets measured at ages of six years and upwards show no appreciable retardation of growth; even triplets are not infrequently well-grown at birth, and the same is occasionally true of quadruplets (fig. 6). Modern collections of measurements, in which the precautions set out above have been increasingly observed, demonstrate (I) that twins and triplets of unlike sex show resemblance in physical measure ments equal to that observed between brothers and sisters by dis tinct births (correlation about 0.5), (2) that pairs of like sex are on the average much more closely alike (correlation about 0.73), (3) that pairs of like sex are heterogeneous in their resemblance and can be interpreted as a mixture of two groups, one with cor relation about 0.5, who may be regarded as two-egg twins, and the other with correlation 0.93, presumably one-egg twins; the latter from between so and 6o% of both the twin and triplet data measured. The exceedingly high correlation indicates that the physical dimensions of the head, trunk and limbs in man are in normal conditions only influenced to about 7% of the total variation by other than heritable causes.
in the collection of statistical data of this kind that the method of collection should not admit in undue proportion those striking cases of apparent inheritance often brought to the notice of the investigator. It is best to attempt to enumerate every case in a circumscribed district, or group of families, in a chosen interval of time. When these precautions are taken it is found that an excess of twins certainly occurs, both on the father's and on the mother's side of the family ; the influence on the father's side being, in the material collected by the writer, apparently con fined to the production of one-egg twinning. The production of two-egg twins seems thus to be influenced only by the mother, and depends greatly upon her age. The frequency increases at least three-fold from the age of 18 to that of 38, and is also prob ably influenced by inherited qualities. The frequency of one-egg twinning depends on the inheritance of the father, and probably also on that of the mother, but it is not known to be influenced by age or environmental factors.