The teakettle might boil forever without producing a steam-engine or the actions of a steam-engine. That which is true of one function is true of all functions, and true of Life, which is the sum of vital activities. 95. It is this haziness which made Agassiz “regret to observe that it has almost become an axiom that identical functions presuppose identical organs. There never was a more incorrect principle leading to more injurious consequences.”[43] And elsewhere he argues that organs can exist without functions. But this is obviously to pervert the fundamental idea of an organ. “The teeth of the whale which never eat through the gums, and the breasts of the males of all classes of mammalia,” are cited by him as examples of such organs without functions; but in the physiological significance of the term these are not organs at all. It is no more to be expected that the breasts of the male should act in lactation, than that the slackened string of a violin should yield musical tones; but the breasts of the male may be easily stimulated into yielding milk, and the slackened string of the violin may be tightened so as to yield tone. Even the breasts of the female do not yield milk except under certain conditions, and in the absence of these are on a par with those of the male. 96. Organized substance has the general properties of Assimilation, Evolution, Sensibility, and Contractility; each of the special tissues into which organized substance is differentiated manifests a predominance of one of these properties. Thus although the embryo-cells all manifest contractility, it is only the specialized muscle-cell which continues throughout its existence to manifest this property, and in a dominant form; the muscle-cell also assimilates and develops, but besides having these properties in common with all other cells, it has the special property of contracting with an energy not found in the others. All cells respire; but the blood-cells have this property of absorbing oxygen to a degree so far surpassing that of any other cell that physiologists have been led to speak of their containing a peculiar respiratory substance. In like manner all, or nearly all, the tissues contain _myeline_--which indeed is one of the chief constituents of the yolk of eggs--but only in the white sheath of the nerves is it detached and specialized as a tissue. 97. But while Sensibility and Contractility are general properties of organized substance, specialized in special tissues; Sensation and Contraction are functions of the organs formed by such tissues; and these organs are only found in animal organisms. It is a serious error, which we shall hereafter have to insist on, to suppose that Sensation can be the property of ganglionic cells, or, as it is more often stated, the property of the central gray matter. Sensation is the function of the organism; it varies with the varying organ; the sensation of Touch not being the same as the sensation of Sight, or of Sound.