576* POPULAR SCIENCE MONTHLY. 



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 preserv- 

 ing 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 in- 

 heritors 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 estab- 

 lished. 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 be- 

 yond 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 occur- 

 ring 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 num- 

 bers above and below the average being assumed equal. In the 

 prehension of the highest branchlets a number of the shorter- 



