PROTECTIVE COLORATION IN THE HOUSE-MOUSE. 473 



adaptation can be pointed to in wliicli we have time data, in 

 which we can fix the maximum possible age of the variety ; but 

 very probably the dark Lepidoptera of the great Factory 

 districts of England present such a case, and owe their existence 

 to the SQioky surroundings. 



For an excellent summary of records of this kind in lasects, 

 see Carpenter (4). 



The study of such cases is a most fascinating one, as, owing 

 to the limited number of factors at work, we may be able to 

 trace, step by step, the causes and their modifying effects upon 

 the species, and so to clear up many points as to the details of 

 the process of elimination known as Natural Selection. 



Papers referred to. 



(1) Thomas, 0. — " On a Wild living Mouse of the Mus musculus 

 group in Portugal." Zoologist, 3rd ser. voi. xx. p. 137, 

 1896. 



(3) Barkext-Hamilton, Gr. E. H. — " On Wild Forms or Sub- 

 species of Mus musculus. ''' Zoologist, 3rd ser. vol. xx. 

 p. 178, 1896. 



(3) Kane, W. F. de V. — " Observations on the Development of 



Melanism in Caviptogramma bilineata." (Report to the 

 Fauna and Flora Committee of the Royal Irish Academy.) 

 Irish Naturalist, 1896, vol. v. p. 74. 



(4) Carpenter, Gr. H. — " Colour Changes in Insects." Natural 



Science, vol. ii. p. 287, 1892. 



EXPLANATION OF PLATE 30. 



Varieties of Mus musculus. 



The dark individual on the right-hand side is No. 4 in table (p. 466), the 

 individual on the left is No. 27 ; that in the centre No. 30. 



