Page:Popular Science Monthly Volume 48.djvu/618

576* man, and must be ever on the alert to kill or snare animals serving for food. To identify distant moving objects as such or such, is therefore essential to the preservation of life. Here is one who, perhaps from some advantageous variation in the forms of the lenses, or in the adjusting muscles, or in the retinal elements, has vision so keen that he recognizes a man, or a lion, or a springbok, when its distance is half a mile greater than that at which other Bushmen can recognize it. What happens? He is enabled the sooner to take measures for his safety, or to make preparations for a hunt; and in either case has an increased chance of preserving life. By his wife, who has but the ordinary keenness of vision, he has children, some of whom, if not all of whom, inherit this peculiarity; and for the same reasons as before, these have, other things equal, better chances of surviving than the rest. If among their descendants some have the peculiarity in an increased degree, if some inherit it in the same degree, and others in smaller degrees in consequence of intercrossing, there will be a tendency, in virtue of the more frequent survival of individuals who are wholly inheritors or partially inheritors, to increase the distance-vision of the tribe: the stirp will spread more than other stirps. So that even were there no other way of establishing a variation save inheritance from a single varying individual, we may see how it will, if of life-saving efficiency, become established.

But there is another way in which variations become established. Creatures inhabiting the same region as the Bushmen furnish an illustration. The general structure of the giraffe is interpretable only as resulting from the co-operation of both factors in the production of species: the selection of variations and the inheritance of acquired characters. But there is one trait of structure attributable to natural selection alone. The giraffe has a prehensile tongue, almost snakelike in form. This it curls round the small branches of trees and pulls them into its mouth. So that, other things equal, a giraffe with an unusually long tongue is able to obtain twigs and clusters of leaves that are beyond the reach of those not similarly endowed; and, when food is scarce, has an advantage. As with the long-sighted Bushman so with the long-tongued giraffe, descendants wholly or partially inheriting the variation will form a prosperous and increasing stirp. But now observe that besides extraordinary variations there are the ordinary variations—variations such as those occurring in the sizes of the hands among ourselves. Let us suppose the average length of the giraffe's tongue to be one foot, and that there are all degrees of greater lengths up to thirteen inches, and all degrees of smaller lengths down to eleven inches: the numbers above and below the average being assumed equal. In the prehension of the highest branchlets a number of the