530 APPENDIX 



sufficiently developed to give one individual sensible advantages over 

 another : the protection will in general be only partial, and the period 

 will vary greatly in length for different individuals. It will be definite 

 only in the sense in which an average is definite : and its essential 

 characteristic is that it is not determined by differences between 

 individuals, but by external causes : so that it is properly represented by 

 a vertical ordinate, MKL in Fig, 19, which cuts all paths at an equal 

 distance from OY. 



43. The destructive limit. Let us now consider the destructive limit, 

 represented by a line, DV, cutting all the paths at an equal distance from 

 the other axis. One criticism which may be made is that this representa- 

 tion is not always appropriate, since the quantity of any character required 

 for escaping destruction may vary during the life-time of the individual. 

 Thus if an animal depended for escape on length of leg, doubtless shorter 

 legs would suffice in its youth to carry it into safety than when it became 

 older and heavier : hence it might be said that the line, DV, ought to 

 slope upwards. But we can make a horizontal line suit such cases by 

 changing the character represented in the diagram. Instead of length of 

 leg, let us take actual speed : then the speed required in all cases would 

 be the same (namely, that which outstrips the enemy), and hence the 

 horizontal line would be appropriate. By choosing the right variable, we 

 can make our diagram suit other cases. 



44. Effect on next generation. We may now consider the effect of 

 this destruction on the next generation. The survivors represented by 

 paths between OVW and OQ in Fig. 19, are stronger in the particular 

 quality than those destroyed, and their children will tend to be like them. 

 We shall have to take account, in this connexion, of sexual and ancestral 

 influences ; but let us omit these in the first instance, for separate con- 

 sideration later. We thus simplify the problem to the case in which 

 children merely tend to resemble their parents. 



45. By a slight extension of the significance of the diagram we can 

 use it to represent the members of one generation in all stages of growth. 

 Thus those destroyed in a generation are represented by the figure 

 VWRK, the survivors by LVWQ, and those who are still being 

 protected by the triangle OKL. These last may in general be dismissed 

 from consideration when we are discussing the next generation : for 

 usually breeding does not commence until the period of protection is 

 over, and the animal is thrown on its own resources. The parents of 

 the next generation will thus be represented simply by the figure LVWQ, 

 and their children will thus tend to follow paths between OW and OQ. 



46. Thus the primary result of destroying the unfit is to improve the 

 next generation. The type-path tends to rise, and will continue to rise 

 as the destruction is repeated until the paths all clear the point V in Fig. 

 1 9 : that is to say the character represented will gradually improve. It 

 will probably go on improving even after the point V is cleared : for V 

 itself will tend to rise, since one animal is often the prey of another, and 

 the struggle between them will tend to improve both. 



47. The point V may also move to the left. For the children of the 



