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call this value P, and write each stature under the form of P + x (in which x includes its sign), and if the number of times with which any value P + x occurs, compared to the number of times in which P occurs, be called y, then x and y are connected by the law of Frequency of Error.

Though the impediments to flight are less unfavourable, on the average, to the stature P than to any other, they will differ in different experiences. The course of one animal may chance to pass through denser foliage than usual, or the obstacles in his way may be higher. In that case an animal whose stature exceeded P would have an advantage over mediocrities. Conversely, the circumstances might be more favourable to a small animal.

Each particular line of escape would be most favourable to some particular stature, and whatever the value of x might be, it is possible that the stature P + x might in some cases be more favoured than any other. But the accidents of foliage and soil in a country are characteristic and persistent, and may fairly be considered as approximately of a typical kind. Therefore those that most favour the animals whose stature is P will be more frequently met with than. those that favour any other stature P + x, and the frequency of the latter occurrence will diminish rapidly as x increases. If the number of times with which any particular value of P + x is most favoured, as compared with the number of times in which P is most favoured, be called y', we may fairly assume that y' and x are