mechanics as a class? They would do so only if the faculties which a capable physician must possess are found among mankind in a limited degree. And mechanics, in turn, would receive wages higher than those of day laborers only if it proved that but a limited number possessed the qualities needed. On this crucial point, to repeat, we are unable to pronounce with certainty. What are the relative effects of nature and of nurture in bringing about the phenomena of social stratification, we cannot say."[32] Next among the facts which account for the existence of relatively separate groups of wage earners are those which are usually summed up under the phrase inequality of opportunity. Equality of opportunity in the way of education and training, and in the way of healthy and strengthening environment would have to be assured before the theory of a general rate of wages could possibly apply. This equality of opportunity is not realized in the United States to-day. The United States has been the scene of continuous and heavy immigration. The mass of this immigration entered into the field of unskilled labor. The great majority of these workers because of the partly unavoidable handicap of their strangeness, and their ignorance of American life, and because of their poor education, did not have equal chances with the older inhabitants to rise in the industrial scale. They could not possibly make the same use of the common opportunities--even if their natural ability were on a par with those of the older inhabitants. Furthermore, the rapid growth of our great cities and the accompanying social changes, the growth in the size of the average industrial enterprise, and the progress of standardization have all lessened equality of opportunity. The chances of the children born in the lowest industrial groups to discover and fairly test their natural abilities have declined in relation to the chances of the children more fortunately born. These conditions have certainly checked the working out of those forces on which the theory of a general rate of wages rests. Thirdly, there is the fact that certain forms of work on which youthful labor is employed, give no preparation and training for the further stages of life and work; and these blind alley employments are filled by children born in the lowest industrial groups.