52 THE POPULAR SCIENCE MONTHLY 



muddier, the broad crabs were subjected to a heavier death rate, so 

 that the general type of crab in the harbor became comparatively nar- 

 row. Selection of this sort may depend on some physical advantage, or 

 sometimes on differences in mentality, as in the survival of races of 

 mankind intelligent enough to overcome a hostile environment — to 

 escape cold by the use of fire or smallpox by the invention of vaccination. 

 The classification of the modes of natural selection is shown by the 

 following diagram: 



Sustentative 



Natural selection 



Non-sustentative 



t, , ,. f Sexual 



Reproductive J 



Fecundal 



An indirect form of sustentative selection may be said to exist 

 when malnutrition has the effect of destroying resistance to an adverse 

 environment, as in the case of tuberculosis. Both modes of lethal selec- 

 tion, moreover, may act upon groups, as well as upon single individuals. 

 Though group selection is best typified by a war between two tribes, 

 such cases as the decline of the Alaskan aborigines illustrate its occur- 

 rence without combat. 



Chapter II. Artificial Selection 



Throughout the measureless ages before man natural selection 

 worked without let or hindrance. 



At last, however, the life of the tree-dwellers evolved that ape-like 

 ancestor of ours, the tailless arboreal mammal with four hands. The 

 hand, originally an adaptation for the purpose of grasping limbs, grad- 

 ually became fitted for grasping other things than limbs, that is, for 

 "handling" tools. Naturally such an organ made slight variations of 

 brain structure of great advantage, for a better way of handling ob- 

 jects in hunting and fighting might lead to survival, not only of the 

 individual, but also of the tribe. The resulting complexity of brain 

 development brought with it the capacity to think, to hand on thoughts 

 from generation to generation by oral and written tradition, and thus 

 to formulate ideas into science. 



Here, however, came a marvelous change in the methods of evolu- 

 tion. Although our primitive ancestors had been, up to this time, like 

 the lower animals, a mere bagatelle under the influences of environment 

 and heredity, thinking man now acquired the power of reflection, and 

 even of discovering and criticizing the laws of evolution themselves. 

 In certain cases we are able to register our dissatisfaction with the 

 course which nature is leading us, and to make definite attempts at 

 turning this course from the merely fittest to our notions of the best. 

 "We have learned that the forces at work changing our species are them- 

 selves partially under our control, and that, if, like the powers of the 



