All of these kinds of causes may overlap, and almost always two or more of them exist in conjunction. Cases where social causes alone account for a migration are rare. They are frequently, however, a contributory factor. The economic causes are by far the most important and universal, though we need frequently to look for other causes back of them. Political maladjustments often express themselves through economic or social disabilities, religious differences through economic and social limitations, etc. In any actual case of migration it is probable that the motives of migration will be due to a complication of causes. This fourfold classification, however, is of great aid in isolating and understanding the underlying forces. The effects of migratory movements, involving the transference of bodies of people from one region to another, are far-reaching and extremely diversified. They concern both the country of origin and the country of destination. They differ widely in specific cases, so much so that it is scarcely possible to lay down any general rules or conclusions which will be of value. They manifest themselves under three main heads, viz. the density of population, the physical stock, and the customs and institutions, or mores. The most obvious effect, and the one which is commonly assumed to follow any migration, is a decrease in the population of the country of source, and an increase in that of the country of destination. But even this, as will appear hereafter, is not by any means the universal rule. There is commonly some effect on the physical stock of the country receiving the migrants. This effect may vary between wide extremes. Whether the customs and institutions shall be also affected depends upon a variety of circumstances which are likely to make each instance distinctive. There is scarcely one of the vital interests of either country concerned which may not be deeply affected by an important migratory movement. But the factors concerned are so complicated, and so subject to individual variation, that movements which bear a general resemblance may have very diverse effects, and each case must be studied by itself. As to the routes or channels of migratory movements, it may be said that in general they follow the lines of least resistance, as determined by the combination of all the forces involved. The closer the movement is to a purely natural one, the more it will follow the natural routes marked out by the configuration of the earth. River valleys, such as the Danube in Europe and the Ohio in America, have always been favorite migratory routes. If mountains have to be traversed, the easiest passes will be chosen, such as the Cumberland Gap in the United States. In general, water has been a bond and not a barrier between different lands, and the earliest routes of distant travel were undoubtedly by water. Greece became the source of numerous migratory movements partly because of her extended coast line.[5]