Evolution Going On 187 



But it is very important to realise that among plant and 

 animals likewise, Evolution is going on. 



The Fountain of Change : Variability 



On an ordinary big clock we do not readily see that even the 

 minute hand is moving, and if the clock struck only once in a hun- 

 dred years we can conceive of people arguing whether the hands 

 did really move at all. So it often is with the changes that go 

 on from generation to generation in living creatures. The flux is 

 so slow, like the flowing of a glacier, that some people fail to be 

 convinced of its reality. And it must, of course, be admitted that 

 some kinds of living creatures, like the Lamp-shell Ligula or the 

 Pearly Nautilus, hardly change from age to age, whereas others, 

 like some of the birds and butterflies, are always giving rise to 

 something new. The Evening Primrose among plants, and the 

 Fruit-fly, Drosophila, among animals, are well-known examples 

 of organisms which are at present in a sporting or mutating mood. 



Certain dark varieties of moth, e.g. of the Peppered Moth, 

 are taking the place of the paler type in some parts of England, 

 and the same is true of some dark forms of Sugar-bird in the West 

 Indian islands. Very important is the piece of statistics worked 

 out by Professor R. C. Punnett, that "if a population contains 

 .001 per cent of a new variety, and if that variety has even a 5 per 

 cent selection advantage over the original form, the latter will al- 

 most completely disappear in less than a hundred generations." 

 This sort of thing has been going on all over the world for untold 

 ages, and the face of animate nature has consequently changed. 



We are impressed by striking novelties that crop up a 

 clever dwarf, a musical genius, a calculating boy, a cock with a 

 10 ft. tail, a "wonder-horse" with a mane reaching to the ground, 

 a tailless cat, a white blackbird, a copper beech, a Greater Celan- 

 dine with much cut up leaves; but this sort of mutation is com- 

 mon, and smaller, less brusque variations are commoner still. 

 They form the raw materials of possible evolution. We are 



