Among these is the influence of customary wage relationships upon the course of wage movements within an industry, and to a lesser extent throughout industry. Because of the existence of vague customary relationships, wage movements affecting some groups or classes of labor are likely to stimulate similar movements among other groups; though it is plain that the efforts of different groups may not meet with equal success. This is well exemplified in the case of railway labor, of which Mr. Stockett has written, "Indeed there is every likelihood that the existence of a powerfully organized and highly paid group of labor in any industry--such as the engineers and conductors in railway transportation--far from being detrimental, may in the long run, be beneficial to the interests of the unorganized and low paid workmen. There is a tendency among the employees to keep a close watch on the wages paid to other groups of their fellow workmen, and the differential between their wage and that of some other grade of employment is jealously guarded. Thus on the railways, wage increases usually advance in cycles, an advance to engineers being followed at a close interval by an equivalent advance to firemen, conductors and trainmen. Existing differentials are more jealously maintained among the train service employees than among other railway workers, but that the latter do aim to maintain their relative level below the skilled groups is evidenced by the reference in arbitration proceedings to the advances made by the train service employees and by their claims to proportionate advances. Thus an increase in the wages of a highly paid group of employees, on account of this tendency to maintain existing differentials tends to put in motion a cycle of wage advances extending to all grades of labor."[36] Public opinion and public agencies of wage settlement have in the past been inclined to give support to the idea of the maintenance of customary relationships, even when the justification was flimsy. Far more important is the factor of mutual aid between groups. For example, in pursuance of some general object skilled groups of labor have given support to minimum wage legislation for unskilled female labor; or again, such instances as the occurrence after the panic of 1907, when various organized groups of wage earners made common cause to resist wage reductions even for unskilled and unorganized labor. Such mutual aid plays its part in determining the wage levels of the different groups of wage earners. This concludes the explanation of the forces which govern the relative wage levels of the separate groups or classes of labor. The actually existing differences of earnings between different groups of labor can only be explained by the combined influence of all the forces discussed.