MATURITY 



/ 



O, 



v 



survivors run through the stages when they need protection more quickly 



than the generation before them. Now .. 



the protection itself is a product of natural 



selection, and will not be maintained 



without reason. Hence as each generation 



needs it for a shorter period (on the 



average) it may shorten automatically. 



Hence O moves to the left and upwards 



from a position such as V 1 to another 



such as V 2 ; and the path representing 



the growth of the character which deter- 



mines destruction or survival will move 



from OPj to OP 2 : that is, its gradient 



M 2 



FIG. 20. 



M, 



will become steeper, at any rate until it 



has surmounted the point V. As to what happens after this we will 



inquire presently. 



48. Now in 15 it was pointed out that this means earlier develop- 

 ment of the organ or character ; and thus we see the effect of natural 

 selection is to make organs on which it acts develop early. 



49. Other organs. Again there is nothing impossible in the associa- 

 tion of an early development of organ Y with retarded development of 

 organ Z. But if Y and Z are both acted on by natural selection there 

 will be a tendency for both to develop early ; and the relative proportions 

 of the animal, so far as these two organs are concerned, will tend to be 

 preserved while at the same time its development is accelerated. But 

 natural selection acts on an immense number of characters in a complex 

 animal, and the same argument will extend to them all. Hence there is 

 a continual tendency to preserve those individuals which run quickly 

 through the copies set them by their parents, at any rate until they reach 

 the destructive limit. 



50. Modification of curve. But now what happens after reaching this 

 We return to the case of a single organ for simplicity. Once the 



limit is attained, and the individual is 

 secure of survival, he may, so to speak, 

 please himself whether he goes further. 

 Those who survive at V may go on 

 to Q or only to R, and their children 

 will be equally numerous. Now we 

 have hitherto dealt with straight paths 

 and regarded such a path as OVR as 

 unusual: but this was before the idea 

 of natural selection was introduced ; and 

 X we shall presently see reasons for modi- 

 fying the results previously arrived at. We 

 shall find in fact that influences tending to depress the portion VQ to VR ; 

 so that after a certain point the growth is slower than at first. This is a well- 

 known feature of the growth of all animals and plants : after a certain point 

 they reach " full growth," and further development is comparatively slow. 



limit ? 

 Y 



FIG. 21. 



