INTRODUCTORY 25 



found to show much or little variability. The very charac- 

 teristic which in one species may exhibit extreme variability 

 may in an allied species show extreme constancy. Illustrations 

 will occur to any naturalist, but nowhere is this truth more 

 strikingly presented than in the British Noctuid Moths. Many 

 are so variable that, in the common phrase, "scarcely two can 

 be found alike," while others show comparatively slight variation. 

 It need scarcely be remarked that, in the instances I have in 

 mind, the evidence of great variability is in no way due to the 

 abundance with which the particular species occurs, for common 

 species may show constancy, and less abundant species may show 

 great variability. The polymorphism seems to be now at least 

 a general property of the variable species, as the fixity is a 

 property of the fixed species. In illustration I may refer to the 

 following examples. 



Dianthoecia capsincola is a common and widely distributed 

 moth which feeds on Lychnis. It shows little variation. Dian- 

 thoecia carpophaga is another species which feeds chiefly on 

 Silene. Its habits are very similar to those of capsincola. Like 

 that species it has a wide geographical range and is abundant 

 in its localities, but in contrast to the fixity of capsincola, car- 

 pophaga exhibits a complex series of varieties. Agrotis stiff usa 

 (= ypsilon) is a moth widely spread through the southern half 

 of England. It is very constant in colour and markings. Ag- 

 rotis segetum and tritici are excessively variable both in ground 

 colour and markings, being found in an immense profusion of 

 dissimilar forms throughout their distribution. Of these and 

 several other species of Agrotis there are many named varieties, 

 some of which have by various writers been regarded as speci- 

 fically distinct. Of the genus Noctua many species (e. g. f estiva) 

 show a similar polymorphism, but N. triangulum, though showing 

 some variation in certain respects, is usually very constant to 

 its type, and the same is true of N. umbrosa. 



In several species of Taeniocampa, especially instabilis, the 

 multiplicity of forms is extreme, while cruda (= pulverulenta) 

 is a comparatively constant species. The genus Plusia contains 

 a number of constant species, but in Plusia interrogationis we 



