Whether the several foregoing modifications would become hereditary, if the same habits of life were followed during many generations, is not known, but it is probable. Rengger (23. ‘Säugethiere von Paraguay,’ 1830, s. 4.) attributes the thin legs and thick arms of the Payaguas Indians to successive generations having passed nearly their whole lives in canoes, with their lower extremities motionless. Other writers have come to a similar conclusion in analogous cases. According to Cranz (24. ‘History of Greenland,’ Eng. translat., 1767, vol. i. p. 230.), who lived for a long time with the Esquimaux, “the natives believe that ingenuity and dexterity in seal-catching (their highest art and virtue) is hereditary; there is really something in it, for the son of a celebrated seal-catcher will distinguish himself, though he lost his father in childhood.” But in this case it is mental aptitude, quite as much as bodily structure, which appears to be inherited. It is asserted that the hands of English labourers are at birth larger than those of the gentry. (25. ‘Intermarriage,’ by Alex. Walker, 1838, p. 377.) From the correlation which exists, at least in some cases (26. ‘The Variation of Animals under Domestication,’ vol. i. p. 173.), between the development of the extremities and of the jaws, it is possible that in those classes which do not labour much with their hands and feet, the jaws would be reduced in size from this cause. That they are generally smaller in refined and civilised men than in hard-working men or savages, is certain. But with savages, as Mr. Herbert Spencer (27. ‘Principles of Biology,’ vol. i. p. 455.) has remarked, the greater use of the jaws in chewing coarse, uncooked food, would act in a direct manner on the masticatory muscles, and on the bones to which they are attached. In infants, long before birth, the skin on the soles of the feet is thicker than on any other part of the body; (28. Paget, ‘Lectures on Surgical Pathology,’ vol. ii, 1853, p. 209.) and it can hardly be doubted that this is due to the inherited effects of pressure during a long series of generations.