Owing to the fertility of inventors and the resultant multiplicity of designs it is impossible to describe every type of heavier-than-air machine which has been submitted to the exacting requirements of military duty. The variety is infinite and the salient fact has already been established that many of the models which have proved reliable and efficient under normal conditions are unsuited to military operations. The early days of the war enabled those of doubtful value to be eliminated, the result being that those machines which are now in use represent the survival of the fittest. Experience has furthermore emphasised the necessity of reducing the number of types to the absolute minimum. This weeding-out process is being continued and there is no doubt that by the time the war is concluded the number of approved types of aeroplanes of military value will have been reduced to a score or less. The inconveniences and disadvantages arising from the utilisation of a wide variety of different types are manifold, the greatest being the necessity of carrying a varied assortment of spare parts, and confusion in the repair and overhauling shops. The methodical Teuton was the first to grasp the significance of these drawbacks; he has accordingly carried standardisation to a high degree of efficiency, as is shown in another chapter. At a later date France appreciated the wisdom of the German practice, and within a short time after the outbreak of hostilities promptly ruled out certain types of machines which were regarded as unsuitable. In this instance the process of elimination created considerable surprise, inasmuch as it involved an embargo on the use of certain machines, which under peace conditions had achieved an international reputation, and were held to represent the finest expression of aeronautical science in France as far as aeroplane developments are concerned. Possibly the German machine which is most familiar, by name, to the general public is the Taube, or, as it is sometimes called, the Etrich monoplane, from the circumstance that it was evolved by the Austrian engineer Igo Etrich in collaboration with his colleague Wels. These two experimenters embarked on the study of dynamic flight contemporaneously with Maxim, Langley, Kress, and many other well-known pioneers, but it was not until 1908 that their first practical machine was completed. Its success was instantaneous, many notable flights being placed to its credit, while some idea of the perfection of its design may be gathered from the fact that the machine of to-day is substantially identical with that used seven years ago, the alterations which have been effected meanwhile being merely modifications in minor details.