When a comparison is made between maternal weight and weight of young in mammals the significant relation is of maternal weight to total weight of young at a birth.
A collection of data for 114 species is presented. The logarithms of maternal and newborn weights have a straight line relation from bats to whales. Relatively the larger mammals carry a smaller weight of young.
Sources of variation in the ratio of weight of young to weight of mother are discussed and a brief comparison is made with the results of Heinroth's analysis of weights of birds and their eggs.