It is not necessary for our present purpose to have any definite theory of the obscure laws by which this variability is governed.[221] It is enough that natural selection requires the striking out of new modifications as well as the transmission of those already produced.[222] It may be the fact that variation is, in the last resort, due to changes in surrounding circumstances, to the unequal incidence of external forces upon a finite aggregate.[223] But, with living bodies as now constituted, it has, at any rate as proximate cause, a twofold source. It may be due to the direct effect of external forces, or it may be caused by the energy stored up in the organism in growth.[224] [Sidenote: consciously directed in man.] In man the outgo of this force is conscious; and, by means of his conscious or intelligent volition, governed by interests of various kinds, he can anticipate and modify the action of natural selection. The law that the fittest organism survives may perhaps work in man as in the lower animals, if only we give a wide enough meaning to "fittest," so as to admit even of the weak being made fit through the sympathy and help of the strong. Natural selection becomes dependent upon variations of a kind different from those in the merely animal world, so that its practical effect may be in some cases apparently reversed. We thus see how it is that even Darwin holds that in moralised societies "natural selection apparently effects but little,"[225] at the same time that we may not be inclined to deny the truth of Schäffle's contention[226] that, although circumstances differ, the law of action remains the same. Schäffle points out how, as we rise in the scale of life, especially as it is manifested in human society, the organisation becomes more delicate, and other than merely natural facts have to be taken account of, so that the fittest to live in the new social and intellectual environment is no longer the man of greatest physical strength and skill.