NATURAL SELECTION 



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he child will under these conditions anticipate the parent by more 

 still. This anticipation, or early development, is of importance as will 

 be seen presently. 



40. Natural selection. We now come to the agency that modifies 

 the above conclusions, natural selection, which works by the destruction of 

 the defective. Let us represent the 



growth of the particular character 

 which, by defect, renders the animal 

 liable to destruction by paths be- 

 tween OQ and OR, in the manner 

 already familiar. Then if the animal 

 does not reach a certain lower limit, 

 OD, in this character, it is liable 

 to destruction for simplicity let us 

 say it is actually destroyed. But 

 this liability does not commence at 



birth: animals are usually protected FIG. 19. 



by their parents, sometimes for long 



periods, after birth. The period of protection may be represented in 

 the time scale by OM, so that the animal is not liable to be destroyed 

 until its path has reached the ordinate MV. If then all who have 

 not reached the limit, OD, are destroyed, the survivors will be those 

 following paths between OVW and OLQ. This is the main argument 

 stated in its most elementary form : but in this form it is scarcely 

 applicable to real life : we must generalize the details. 



41. Period of protection. First let us consider the period of parental 

 protection, which is often evanescent. If the argument were no longer 

 applicable in such cases, we should have a serious difficulty. But 

 parental protection is only one form of a more general protection in 

 which chance plays a conspicuous part. Let us first consider a number 

 of seeds scattered on the ground and liable to be eaten by birds. A seed 

 may be snapped up directly it is scattered, or it may never be eaten at 

 all : and though there is thus no definite protection, there will, according 

 to the laws of probability, be an " expectation of life " of a seed an 

 average period for which it may expect to survive. And this period will 

 not be determined by inherent qualities of the seed, but by external 

 circumstances, such as the number of depredating birds, the nature of 

 the soil, and the skill of the sower. There is something very similar in 

 the case of young animals too feeble to depend on their own exertions for 

 escape from destruction. In later life an animal may depend on its 

 speed fo escape, and the speediest will have advantages over those less 

 speedy : but the speed of the very young may not suffice in any case to 

 protect them : slow and speedy may be open to similar risks, like two 

 seeds or two eggs in the same nest. 



42. Hence the notion of the period of protection when generalized 

 will be somewhat complex : it will include not only the period before 

 birth (for viviparous animals), or the period in the egg and in the nest 

 after hatching : but also an additional period before the powers are 



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