Still, however, the fastening would be accessible to everyone, and in a condition of society in which property must always have been insecure, it would become a great desideratum to construct a bolt which could be drawn back only by the use of a key, which the owner might carry about with him, and thereby secure his goods and chattels whilst he himself was absent in the fields, or in the hunting grounds. So necessary a requirement of every day life must have forced itself upon the notice of the greater part of mankind, and it is not surprising, therefore, to find that this stage of the development of the lock forms the point of trifurcation of three separate branches of improvement. Two of these are of the nature of tumbler locks, and consist of apparatus for raising the pin or pins by which the bolt is secured when they fall into the holes provided for them on the upper surface of it. It was for this reason that they were termed _tumblers_, because they tumble into the holes when the lock is closed. The third branch led off in another direction. In order that the mind may not wander from the lines of continuity whilst I treat each of these three branches separately, I shall class them as A, B, and C in the diagrams, at the same time allowing the numbers of the figures to run on continuously from this point of departure. By this means I shall be best able to show the ramifications into which this mechanism, like all similar contrivances to which these papers relate, separate as they increase in complexity. The common door bolt (figs. 2 and 3, Plate I.) having continued to be available as an inside fastening, in addition to more complex contrivances for securing doors, has continued to be universally employed up to the present time, and may be compared in nature to those fossil species, which, having never become unsuited to their environment, have survived throughout successive geological periods, whilst the forms represented in figs. 4 to 8, Plate I., being makeshifts, have disappeared as soon as they were superseded, and thus they constitute the "missing links" of our developmental series.