de Beer • The Darwiri-Wallace Centenary 



283 



many of the products of the plant and 

 animal kingdom convey to man the 

 aesthetic quality of beauty, it has been 

 supposed that beauty is an end in itself 

 to which the criterion of usefulness and 

 survival value could not be applied, 

 and therefore that it could not be im- 

 agined as a product of evolution. To 

 this argument Wallace opposed the 

 demonstration that if the quality of 

 beauty were an exception to the prin- 

 ciple of evolution bv natural selection, 

 it would be necessary to find an expla- 

 nation for the existence of so much in 

 plants and animals that is positively 

 ugly. 



Danvin showed it to be an invari- 

 able rule that "When a flower is fer- 

 tilised by the wind it never has a gaily- 

 coloured corolla." The beauty of 

 flowers has been gradually achieved be- 

 cause of the survival value of cross 

 fertilization (consequent upon the at- 

 traction of insects to such flowers ) con- 

 ferred on plants possessing them. The 

 beautiful colors and structures of birds 

 and some other animals have resulted 

 from the survival value conferred on 

 successful competitors in sexual selec- 

 tion. 



NATURAL SELECTION IN ACTION 



Natural selection can be seen to be 

 at work here and now in directing evo- 

 lution. An example of this t}'pe of re- 

 search is that of H. B. D. Kettlewell on 

 "industrial melanism" in moths. Up to 

 1850 the British peppered moth existed 

 in its typical gray form known as 

 Biston betularia, which is remarkablv 

 well adapted to resemble the lichens 

 on the bark of trees. From that date a 

 dark melanic variety appeared, known 

 as carbonaria, which is extremely con- 

 spicuous against the natural bark of 

 trees. The melanic variation is con- 

 trolled by a single dominant Mende- 

 lian gene and is slightly more vigorous 

 than the normal gray t)pe. Never- 



theless, because of its conspicuous color 

 the carbonaria variet}' was constantly 

 eliminated, and this variety persisted 

 in the populations of the peppered 

 moth only because the same mutation 

 kept on occurring again and again. The 

 industrial revolution brought about a 

 marked change in the environment, 

 since the pollution of the air by in- 

 creasing quantities of carbon dust 

 killed the lichens on the trees and 

 rendered their trunks and branches 

 black. Under these conditions it is the 

 carbonaria variety which is favored and 

 the betularia penalized. This has been 

 proved by direct observation of the 

 feeding of birds, and by measurement 

 of the survival rates of the different 

 forms in the different environments. 

 The dark carbonaria form survives 17 

 per cent less well in an unpolluted area 

 and 10 per cent better in a polluted 

 area. One hundred years ago the dark 

 variety of the peppered moth formed 

 less than 1 per cent of the population; 

 today in industrial areas it forms 99 

 per cent, and selection has made it 

 more intensively black than when it 

 first appeared. 



The case of melanism in the pep- 

 pered moth also introduces a principle 

 to which L. Cuenot drew attention and 

 gave the name of "preadaptarion." The 

 melanic form of the peppered moth 

 happened to be "preadapted" to con- 

 ditions which were only subsecjuentlv 

 realized, or in other words, if the in- 

 dustrial revolution had not taken place, 

 the melanic variety would never have 

 become adaptive at all, and would have 

 suffered the same fate as the countless 

 other mutations resulring in variarions 

 which, whether "preadapted" or not, 

 have been eliminated because they fell 

 short of the requirements imposed by 

 natural selection. 



While the overriding importance of 

 the eflE^ects of selection is now generally 

 realized, it has been suggested that 

 when populations are split up into very 



