Surely my critics cannot be ignorant of the prominent part which imagination has recently played in the exactest of all natural sciences--physics? Are they unaware that the English physicist Maxwell "constructed from liquid vortices and friction-pulleys enclosed in cells with elastic walls, a wonderful mechanism, which served as a mechanical model for electromagnetism"?[2] He hoped "that further research in the domain of theoretical electricity would be promoted rather than hindered by such mechanical {5} fictions." And so it actually happened, for Maxwell found by means of them "the very equations, whose singular and almost incomprehensible power Hertz has so beautifully portrayed in his lecture on the relations between light and electricity." "Maxwell's formulae were the direct outcome of his mechanical models." "These ideal mechanisms"--so relates Boltzmann in the same interesting essay--"were at first widely ridiculed, but gradually the new ideas worked their way into all fields. They were themselves more convenient than the old hypotheses. For the latter could be maintained only in the event of everything's proceeding smoothly; whereas now little inconsistencies were fraught with no peril, for no one can take amiss a slight hitch in a mere analogy.--Ultimately Maxwell's ideas were philosophically generalised as the theory that all knowledge consists in the disclosure of analogies." But not only does it seem that there is little appreciation among biologists for the scientific import of imagination, they also appear to have little sense for the significance of theory. It is a favorite attitude nowadays to look upon theory as a sort of superfluous ballast, as a worthless survival from the epoch of decrepit "nature-philosophies." People pronounce with pride the miscomprehended utterance of Newton, _Hypotheses non fingo_, and place the value of the slightest new fact infinitely higher than that of "the most beautiful theory."[3] And yet theory originally {6} fashions science out of facts and is the indispensable precondition of every important scientific advance.