NATURAL SELECTION 193 



himself, Natural Selection appeared as an inevitable conse- 

 quence of certain satisfactorily established phenomena, viz. 

 numerical multiplication, competition, etc. He did not pro- 

 duce evidence for the actual occurrence of a differential 

 death-rate. 



Pearl (1930) has set out concisely the requirements of a 

 proof that Natural Selection has altered a race. These are : 



(a) Proof of somatic difference between survivors and 



eliminated. 



(b) Proof of genetic differences between survivors and 



eliminated. 



(c) Proof of effective time of elimination. 



(d) Proof of the somatic alteration of the race. 



(e) Proof of the genetic alteration of the race. 



(c) implies that selection must occur before reproduction 

 is complete. 



As will be seen from the examination of the direct evidence 

 (pp. 196-212), most of the investigators have concerned them- 

 selves with (a) only. 



Before considering the evidence that a selective process is 

 or is not actually at work, certain general considerations as to 

 the death-rates of animals in nature may be brought forward. 

 Thompson and Parker (1928) in their study oiPyrausta ?iubilalis, 

 the European Cornborer, find that at least 90 per cent, of the 

 young larvae are killed off before any predators or parasites 

 have begun their attack. According to these authors, ' more 

 individuals disappear because of their highly restricted adaptive 

 powers than through all the other controlling factors taken 

 together.' The young larvae are extremely delicate. If they 

 fall to the ground or into a drop of water, or if they emerge 

 when the food-plant is too hard, they are likely to die. A 

 slight injury or deprivation of food for a short period causes 

 a high mortality. In a rapidly fluctuating environment many 

 larvae, even though on the whole better adapted than their 

 neighbours, must succumb without a chance of justifying 

 themselves. 



Salt recently (1 931), in a very careful study of the Wheat- 

 stem Sawfly (Cephas pallipes) , found that a part only of the larval 

 mortality accounted for 89 per cent, of the pre-adult individuals. 

 Thorpe (1930a) found in the Pine-shoot Moth (Rhyacionia 



