Without further argument, it will, we think, be sufficiently clear that the sciences are none of them separately evolved—­are none of them independent either logically or historically; but that all of them have, in a greater or less degree, required aid and reciprocated it.  Indeed, it needs but to throw aside these, and contemplate the mixed character of surrounding phenomena, to at once see that these notions of division and succession in the kinds of knowledge are none of them actually true, but are simple scientific fictions:  good, if regarded merely as aids to study; bad, if regarded as representing realities in Nature.  Consider them critically, and no facts whatever are presented to our senses uncombined with other facts—­no facts whatever but are in some degree disguised by accompanying facts:  disguised in such a manner that all must be partially understood before any one can be understood.  If it be said, as by M. Comte, that gravitating force should be treated of before other forces, seeing that all things are subject to it, it may on like grounds be said that heat should be first dealt with; seeing that thermal forces are everywhere in action; that the ability of any portion of matter to manifest visible gravitative phenomena depends on its state of aggregation, which is determined by heat; that only by the aid of thermology can we explain those apparent exceptions to the gravitating tendency which are presented by steam and smoke, and so establish its universality, and that, indeed, the very existence of the solar system in a solid form is just as much a question of heat as it is one of gravitation.

Take other cases:—­All phenomena recognised by the eyes, through which only are the data of exact science ascertainable, are complicated with optical phenomena; and cannot be exhaustively known until optical principles are known.  The burning of a candle cannot be explained without involving chemistry, mechanics, thermology.  Every wind that blows is determined by influences partly solar, partly lunar, partly hygrometric; and implies considerations of fluid equilibrium and physical geography.  The direction, dip, and variations of the magnetic needle, are facts half terrestrial, half celestial—­are caused by earthly forces which have cycles of change corresponding with astronomical periods.  The flowing of the gulf-stream and the annual migration of icebergs towards the equator, depending as they do on the balancing of the centripetal and centrifugal forces acting on the ocean, involve in their explanation the Earth’s rotation and spheroidal form, the laws of hydrostatics, the relative densities of cold and warm water, and the doctrines of evaporation.  It is no doubt true, as M. Comte says, that “our position in the solar system, and the motions, form, size, equilibrium of the mass of our world among the planets, must be known before we can understand the phenomena going on at its surface.”  But, fatally for his hypothesis, it is also true that