224 Ge7ietical Studies in Moths 



be seen that the pinewood race or races are derived from those in the 

 birchwood by a process of selection working against the pale silvery 

 insects which one looks upon as type 0. autumnata. 



Let us proceed to investigate the mechanism of this limitation. In 

 the birchwood the silvery insects do most certainly blend best with the 

 trunks upon which they rest ; so, too, do the darker insects of the other 

 wood with the dull-toned larch bark. But as a protection in the daytime 

 this would be valueless should they be molested by tits or other insec- 

 tivorous birds and like enemies. All of the insects, pale, medium or 

 dark, at once betray themselves by flying off the trunks, even when the 

 disturbing agent is yards away. Selection thus would not be preferen- 

 tial. Except for a solitary pair of wood pigeons, and once a thrush, the 

 birchwood has to me always seemed void of bird life ; matters, however, 

 are different in the pines. There birds are much more prevalent, bull- 

 finches and other birds abound, but during all my years of observation I 

 have never seen a single Oporabia of any species attacked by birds by 

 day! At night time the position changes; amongst the birches few if 

 any nocturnal birds occur, for the trees are not well enough grown to 

 harbour them. In the pines hosts of owls and night-jars aided by bats 

 wreak terrific havoc on the insect life of the wood, as the number of 

 lepidopterous wings and coleopterous elytra lying about show. 



Recognising this fact, I have from time to time critically examined 

 all of the detached 0. autumnata forewings I have seen in the wood. In 

 all I have noted 17 ; two of these, both dark, were entangled in the 

 floccose web of the very abundant spider Amaurobius fenestralis, and 

 15 lay on the grass — in some cases in pairs. Those taken by the spider 

 are negligible; chance ensnarement explains them. The other 15, in 

 a population where the dark individuals outnumber the pale by more 

 than 25 to 1, actually included a majority of pale wings ! This cannot 

 be a mere matter of chance ; selection must be at work — and natural 

 selection carried out by bats, owls and night-jars. These in the uncertain 

 twilight of the wood at the flight-time of the moth would more readily 

 secure the more conspicuous paler strains, thus progressively eliminating 

 them. The observation that the paler examples yet existing were more 

 heavily barred than the bulk of the parallel birch forms suggests that 

 even the barred birch form itself is not an elementary strain, but was 

 capable of movement in the mode when submitted to selection, either 

 natural or artificial. 



In my opinion, therefore, it is to natural selection, carried on by 

 nocturnal birds in company with bats, we have to look to explain the 



