The general nature of Weismann’s explanation may be summed up in a curious illustration given by him in The Evolution Theory.[68] Let us suppose, he says, a snow-field surrounded by precipices on all sides, but with a narrow track leading away from it at one point. Scattered about on the snow-field are a number of persons. A sleigh is now projected among them from some outside point. Each person, when the sleigh comes near him, gives it a push, but he has no object in pushing it anywhere in particular, and simply sends it flying off in whatever direction he chances to be looking. What will happen under these circumstances? After more or less bandying about, the sleigh will, in the vast majority of cases, fall into one of the abysses round the snow-field and be lost But another is then launched on to the snow-field, and then another and another without end; and so, at last, it may happen that a series of pushes will take place which will send the sleigh over the narrow track to its goal. The goal is supposed to represent some condition to which the organism (the sleigh) has to adapt itself. The random pushes which it receives are the multitude of variations constantly occurring in the reproductive cells. Most of these variations have no decisive tendency, favourable or unfavourable. If a series of unfavourable ones should occur, leading to some development which markedly impairs the chances of the organism for success in life, it, or its line of succession, dies out, and the unfavourable variation is, therefore, not perpetuated. This is illustrated by the sleigh going into the abyss. But if a favourable variation occurs, and is increased till it reaches ‘selection value,’ _i.e._ till it gives the organisms possessing it a distinct advantage over others in the battle of life, then this favoured type will ultimately, by the action of natural selection, drive out the less favoured, and will establish itself as the sole representative of the species. Having reached this level, of course the same process will go on further indefinitely.