OLD AGE AND NATURAL DEATH 



selection.* But before looking into this belief more closely, 

 it will be as well to start this section, like its less technical 

 predecessor, with a few definitions. 



First, ''evolution\ Biologists often speak of organs, tissues 

 and even cells *"evolving% but it must be recognized that this 

 manner of formulation is by modern lights imprecise, or, what 

 is not quite the same thing, inexpedient. These various things 

 do indeed participate in evolution, just as our noses participate 

 in our motion without themselves being mobile. What moves in 

 evolution, what evolves, is an animal population, not an indi- 

 vidual animal; and the changes that occur in the course of 

 evolution, if we put a magnifying glass to them instead of 

 ' feeling obliged to peer dimly down the ages of geological time, 

 are changes in the composition of a population and not, prim- 

 arily, in the properties of an individual. In visual analogy they 

 are to be likened, not to a transformation scene at the panto- 

 mime, but to the sort of overlapping transformation we watch 

 at the cinema when one ""sef slowly evaporates and is dis- 

 possessed of the screen by another. 



Further, whatever form evolution may take, or whatever 

 may bring it about, contributions to evolutionary change are 

 paid, if they are paid at all, in one currency alone: offspring. 

 Animals favoured by the process which, wise after the event, 

 we call ^'natural selection"*, pay an extra contribution, however 

 small, to the ancestry of future generations; and this brings 

 about just that shift in the genetical composition of a popula- 

 tion which we call an '"evolutionary change\ The problem of 

 measuring natural selection, which so worried Karl Pearson,^ 

 is thus solved: the magnitude of natural selection is measured 

 by the relative increase or decrease in the frequency with 



1 Cf. K. Pearson, The Chances of Death and Other Studies in Evolution, 

 2 vols., London, 1897. 



* [The argument sketched in this section is developed more fully in the 

 essay which follows, An Unsolved Problem of Biology.^ 



S5 



