The truth is, that the classes of ships which constitute a fleet are, or ought to be, the expression in material of the strategical and tactical ideas that prevail at any given time, and consequently they have varied not only with the ideas, but also with the material in vogue. It may also be said more broadly that they have varied with the theory of war, by which more or less consciously naval thought was dominated. It is true that few ages have formulated a theory of war, or even been clearly aware of its influence; but nevertheless such theories have always existed, and even in their most nebulous and intangible shapes seem to have exerted an ascertainable influence on the constitution of fleets. Going back to the dawn of modern times, we note that at the opening of the sixteenth century, when galley warfare reached its culmination, the constitution was threefold, bearing a superficial analogy to that which we have come to regard as normal. There were the galeasses and heavy galleys corresponding to our battleships, light galleys corresponding to our cruisers, while the flotilla was represented by the small "frigates," "brigantines," and similar craft, which had no slave gang for propulsion, but were rowed by the fighting crew. Such armed sailing ships as then existed were regarded as auxiliaries, and formed a category apart, as fireships and bomb-vessels did in the sailing period, and as mine-layers do now. But the parallel must not be overstrained. The distinction of function between the two classes of galleys was not so strongly marked as that between the lighter craft and the galleys; that is to say, the scientific differentiation between battleships and cruisers had not yet been so firmly developed as it was destined to become in later times, and the smaller galleys habitually took their place in the fighting line.