"From the time when the social and sympathetic feelings came into operation and the intellectual and moral faculties became fairly developed, man would cease to be influenced by natural selection in his physical form and structure. As an animal he would remain almost stationary, the changes in the surrounding universe ceasing to produce in him that powerful modifying effect which they exercise on other parts of the organic world. But, from the moment that the form of his body became stationary, his mind would become subject to those very influences from which his body had escaped; every slight variation in his mental and moral nature which should enable him better to guard against adverse circumstances and combine for mutual comfort and protection would be preserved and accumulated; the better and higher specimens of our race would therefore increase and spread, the lower and more brutal would give way and successively die out, and that rapid advancement of mental organisation would occur which has raised the very lowest races of man so far above the brutes (although differing so little from some of them in physical structure), and, in conjunction with scarcely perceptible modifications of form, has developed the wonderful intellect of the European races" (pp. 316, 317, Second Edition, 1871). This argument has suggestive illustration in the fifth chapter of the Origin of Species. Mr. Darwin there refers to a remark to the following effect made by Mr. Waterhouse: "_A part developed in any species in an extraordinary degree or manner in comparison with the same part in allied species tends to be highly variable._" This applies only where there is unusual development. "Thus, the wing of a bat is a most abnormal structure in the class of mammals; but the rule would not apply here, because the whole group of bats possesses wings; it would apply only if some one species had wings developed in a remarkable manner in comparison with the other species of the same genus." And when this exceptional development of any part or organ occurs, we may conclude that the modification has arisen since the period when the several species branched off from the common progenitor of the genus; and this period will seldom be very remote, as species rarely endure for more than one geological period.