202 THE VARIATION OF ANIMALS IN NATURE 



(6) Kane {i8g6) : melanic forms of Camptogramma bilineata. 



Kane found that by 1892 the melanic form, var. isolata, 

 of this species had entirely superseded the lighter-coloured 

 typical form on Dursey Island, Ballinskelligs Bay, co. Kerry. 

 He points out that in this area the cliffs and islands are of a 

 dark slate formation. They are ' haunted by Rock Pipits, 

 Wheatears, Bats and small Gulls (all insectivores).' In 1893- 

 1894 there was a great destruction of the Silene on which the 

 moth lived, and a potential increase in the intensity of destruc- 

 tion — so much indeed that in this period the species virtually 

 became extinct. He thinks that the dark form under intense 

 competition was favoured by its colour (as against the dark 

 background). Some additional evidence is supplied from a 

 study of the distribution of dark forms on heather and peat, 

 and more especially of the prevalence of light forms on the 

 pale grey limestones of co. Clare. Other species tend to show 

 a parallel variation in relation to habitat. Some of the Dursey 

 Island melanics were shown to be of a fixed heredity (Kane, 

 I.e. 1897, p. 44). 



There is no actual evidence as to the discrimination by 

 the alleged enemies. The author attempts to get round the 

 traditional explanation that the occurrence of melanism is 

 correlated with rainfall. We require far more evidence as to 

 the selective value of the dark colour and of the discrimination 

 by the alleged enemies. 



(7) di Cesnola {1904) and Beljajeff {1927) : experiments with 

 Mantis religiosa. 



di Cesnola conducted his experiment as follows : 



20 green Mantis were tethered to green plants. 

 25 „ » » » » brown „ 



20 brown „ „ „ „ „ „ 



45 » » " » » green ,, 



The insects were left exposed for 1 7 days. At the end of that 

 period the 40 ' harmonising ' insects had all survived. Of 

 the 25 ' green on brown ' all had been killed (20 certainly by 

 birds) ; of the 45 ' brown on green ' 10 only were left. All 

 the rest were killed by birds. 



He concludes that the ' concealing ' background does 

 discriminate one type from the other. 



