These processes may be compared to a man on a journey who proceeds from a certain point on foot by short stages, at any given time, and in any direction. He has then the choice of an infinite number of routes over the whole earth. If such a man begins his wanderings in obedience to the impulse of his own will, his own pleasure or interest,—proceeding forwards, to the right or left, or even backwards, with longer or shorter pauses, and starting at any particular time,—it is obvious that the route taken lies in the man himself and is determined by his own peculiar temperament. His judgment, experience, and inclination will influence his course at each turn of his journey, as new circumstances arise. He will turn aside from a mountain which he considers too lofty to be climbed; he will incline to the right, if this direction appears to afford a better passage over a swollen stream; he will rest when he reaches a pleasant halting-place, and will hurry on when he knows that enemies beset him. And in spite of the perfectly free choice open to him, the course he takes is in fact decided by both the place and time of his starting and by circumstances which—always occurring at every part of the journey—impel him one way or the other; and if all the factors could be ascertained in the minutest detail, his course could be predicted from the beginning. Such a traveller represents a species, and his route corresponds with the changes which are induced in it by natural selection. The changes are determined by the physical nature of the species, and by the conditions of life by which it is surrounded at any given time. A number of different changes may occur at every point, but only that one will actually develope which is the most useful, under existing external conditions. The species will remain unaltered as long as it is in perfect equilibrium with its surroundings, and as soon as this equilibrium is disturbed it will commence to change. It may also happen that, in spite of all the pressure of competing species, no further change occurs because no one of the innumerable very slight changes, which are alone possible at any one time, can help in the struggle; just as the traveller who is followed by an overpowering enemy, is compelled to succumb when he has been driven down to the sea. A boat alone could save him, without it he must perish; and so it sometimes happens that a species can only be saved from destruction by changes of a conspicuous kind, and these it is unable to produce.