EVOLUTION OF MAN AND ITS CONTROL 51 



that potato beetles subjected to hot, dry conditions were made lighter 

 in color and that part of their progeny raised under normal environ- 

 ment retained this characteristic. 



Mere modifications of muscle or brain, accordingly, as in the trained 

 mind of a savant or the brawny arms of a blacksmith, are not inherit- 

 able, but such is not necessarily the case with a quality like smallness 

 of stature due to under-feeding, an influence affecting the mechanism 

 of inheritance itself. 



By far the best known of the factors working upon the lower ani- 

 mals is natural selection; that evolution which takes place when, with- 

 out conscious selective action, one generation has been contributed by a 

 part of the previous one differing from the non-contributing portion, as 

 when wild deer of one generation are descended only from those previ- 

 ously existing deer who have been able to live to maturity because of 

 their superior swiftness. Variations are thus seized upon by natural 

 selection and perpetuated by heredity. The evidence of such a selec- 

 tion in the case of man has been tangibly presented by Sir Francis 

 Galton, Karl Pearson and others working with them. 



Natural selection stands opposed to artificial selection, that which is 

 accomplished by conscious effort, such as Burbank's famous work in the 

 production of improved varieties of fruit. It consists of two main proc- 

 esses, each containing several subdivisions, and a common error has 

 been to confine the term natural selection to the first of these main proc- 

 esses, which may be called lethal selection. The case of the deer men- 

 tioned above is an instance of lethal selection — that which results from 

 the death of some individuals before reproduction is completed. 



The second process of natural selection, of perhaps even more im- 

 portance at present in man, is reproductive selection, the result, not of 

 premature death, but of a differential number of progeny. This may 

 sometimes counterbalance lethal selection, as in the survival of some 

 of the lower species of animals, whose marvelous reproductive ability 

 preserves them in the face of a very high death rate. 



When the cause of the absence of progeny is a failure in the indi- 

 vidual to mate at the proper time, it is called sexual selection, but if, for 

 other causes than success in mating, the number of offspring varies 

 among individuals, fecundal selection results. 



Sustentative selection, the type of natural selection most commonly 

 recognized, comes from a pressure upon the means of subsistence by 

 proportionately excessive numbers, such as that which sent the succes- 

 sive waves of Aryan migration over Europe. A non- sustentative form 

 of natural selection takes place from the destruction of the individual 

 by some adverse feature of the environment, such as excessive cold, 

 bacteria or some bodily deficiency, and is independent of mere food- 

 supply. As Plymouth Harbor, for example, kept growing gradually 



