Il8 YORKSHIRE NATURALISTS' UNION. 



fails to suggest that the conditions of existence in those two 

 distinct settlements being approximately the same, a natural law- 

 producing certain forms in the one place might be equally 

 operative to produce them in the other. 



Admitting the extreme value and interest of these re- 

 searches, and allowing them their due weight in the collection of 

 evidence upon the general subject, we may remark that their 

 authors in no way profess to account for the primary cause of 

 the melanic tendency, except in so far as they would admit it to 

 be traceable to the external influence of climatic conditions. 

 We may yet ask ourselves what is the exact method by which 

 the pigments are acted upon, and what advantages, if any, do 

 the insects derive from the change. 



If it is the inferior quality or scarcity of food which 

 influences the pigments of the scales, we may be driven to 

 accept Mr. G. Lewis' statement that ' The nutritive properties 

 in plants vary as their growth is vigorous or otherwise ' ; * but, 

 surely, this might land us in a region of fallacy where the 

 largest insects would invariably be found on the tallest trees, 

 and the most minute upon vegetation of the lowest growth. 



Starvation does undoubtedly produce abnormal forms, 

 frequently devoid of the usual richness or extent of colouring, 

 but in these the physical strength and development are 

 unnaturally affected, and it might reasonably be expected that 

 the process of ornamentation would be also arrested. Proof is 

 yet wanting that larvae feeding on lichens or plants of low growth 

 are less vigorous and healthy, or produce less perfect moths 

 than those which may be supposed to be more generously 

 nourished. 



Retarded development dependent upon climatic conditions 

 must constantly occur in extreme northern or Alpine regions. 



* Transactions of Entomological Society, London, 1882, p. 527. 



Trans.Y.N.U., 1883 (pub. 1S85). Series D 



