THE ORIGIN OF VARIATION 53 



temperate countries in the many insects which have two or 

 more broods a year. In these the pupae from the early broods 

 produce adults in the same year, whereas the pupae of the 

 last brood hibernate. It is difficult to imagine how any such 

 system could keep in step with climatic seasonal changes, if it 

 were not ultimately controlled by temperature or some other 

 climatic variable. When, however, the determinative factors 

 are investigated experimentally, a very perplexing state of 

 affairs is laid bare, recalling in detail the complex problem of 

 seasonal variation in colour. There is little doubt that the 

 gradual sinking of the mean temperature in the autumn is the 

 main controlling factor. Pupae which have been exposed to 

 such a gradual cooling tend to become dormant. But even in 

 one family (of brothers and sisters) the effect is not uniform ; 

 in a number of experiments some individuals become dormant, 

 while others do not. Probably genetic factors partly determine 

 the response to temperature, but Dawson was unable to find 

 any simple scheme of segregation. Previously Toyama (1912), 

 in the Silkworm {Bombyx mori), had suggested matroclinous 

 inheritance. 



In the Cornborer (Pyrausta nubilalis), Babcock (1927) and 

 Babcock and Vance maintain that ' the seasonal rhythm 

 is to a certain extent persistent and is due to the formation 

 of a physiological condition which forces the insect to 

 develop a certain type of seasonal cycle. This physiological 

 condition is formed by continued impress of a particular type 

 of normal environment and persists after the impress of the 

 environment is removed ' (1929, p. 53). The whole question 

 of seasonal rhythms in animals is still in urgent need of 

 experimental investigation. 



Since the genesis of instinct is still so obscure there is some 

 value in putting on record a number of instances of aberra- 

 tions in instinct. Some of these appear to be merely individual, 

 but others have been more widely manifested. In birds and 

 mammals, where social tradition has some weight, even 

 individual aberrations have importance. 



Insects. — One of the best known instances of a sudden 

 change in habits is that of an English bug, Plesiocoris rugicollis, 

 which, prior to 191 8, was known to feed only on willow, but 

 since that date has increasingly turned its attention to apple, 

 so that it is now a serious pest. The flies which ' blow ' sheep 



