Latterly much attention has been attracted, and rightly so, to the number, variety, and potency of the socio-economic factors which strongly incline married couples deliberately to restrain their fertility and thereby to limit the size of-their families. A tendency to overlook a number of purely biological factors which work in the same direction is to be discerned. These latter are for the mos...

IN I804 Baronio clearly demonstrated that experimental free skin autografts in a sheep would survive. The wide variety of free autografts now used in plastic surgery are ample testimony to his findings. Skin grafts exchanged between identical twins were first shown to survive by Bauer in I927. Tissue transplants between members of highly inbred mouse strains are similarly accepted. A kidney tha...