350 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
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 percent better in a polluted 
area. One hundred years ago the dark variety of the peppered moth 
formed less than 1 percent of the population; today in industrial 
areas 1t forms 99 percent, and selection has made it more intensively 
black than when it first appeared. 
The case of melanism in the peppered moth also introduces a prin- 
ciple to which L. Cuénot drew attention and gave the name of “pre- 
adaptation.” ‘The melanic form of the peppered moth happened to be 
“preadapted” to conditions which were only subsequently realized, or 
in other words, if the industrial 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 resulting 
in variations which, whether “preadapted” or not, have been elimi- 
nated because they fell short of the requirements imposed by natural 
selection. 
The evolutionary change actually witnessed in the peppered moth 
is directly attributable to selection, and it is matched by similar 
studies on other forms. Experiments by A. J. Cain and P. M. Shep- 
pard on the survival rate of snails with shells of different colors and 
banding patterns, living on dark- or light-colored backgrounds, have 
shown that selection does not act like an all-obliterating steamroller 
going in one direction. As the seasons change, the adaptive value of 
the color of a shell changes from disadvantageous to advantageous and 
back again. This proves that the effects of selection vary from place 
to place and from season to season, and that the balance between an 
organism and its environment is delicate, changing, and dynamic. 
The phenomenon of Batesian mimicry has also been proved not 
only to be adaptive and to confer survival value, but to have been 
achieved by selection. Ford has shown that the degree of perfection 
with which the mimics copy their models is a function of the prev- 
alence of the models. The percentage of imperfect mimics in the 
populations of Papilio dardanus is only 4 at places like Entebbe, 
where models are numerous. At Nairobi, on the other hand, where 
the models are 70 times less numerous than at Entebbe, the imperfect 
mimics are 8 times more numerous and constitute 32 percent of the 
population. Less survival value is conferred by resemblance to a 
model when the latter is too infrequent to teach predators to shun it, 
and there is then less selection pressure on the mimic to resemble it. 
While the overriding importance of the effects of selection is now 
generally realized, it has been suggested that when populations are 
