XVI INTRODUCTION. 



duction of these varieties was accompanied by a failure to produce 

 fertile eggs. Dr. Chapman connected it with a change of constitution 

 and did not consider it necessarily the result of disease. 



Food is another factor in producing variation, but it is doubtful 

 whether direct phy tophagic varieties are ever anything but variations in 

 size. With plenty of succulent and nutritious food, lepidoptera attain a 

 large size and it is a common occurrence to find races of the same species, 

 of different sizes, living in localities comparatively near to each other; one 

 race, living on more succulent food, of large size, the other, living on 

 less succulent or less nutritious food, of small size. This is especially 

 noticeable in certain GEOMETRY as Hypsipetes elutata, Cidaria testata, 

 Eupithecia satyrata and its var. callunaria and many others. Among 

 the NOCTUJE, I cannot point to a striking example in this direction, 

 although undoubtedly such exist, and in confinement if larvae be kept 

 short of food, dwarfs are the natural result. Generally speaking, the 

 larger races of such species as these, appear to be more clearly marked, 

 but this I do not consider in any way due to food, but to the combined 

 influences of " natural selection " and to the fact that the dark mark- 

 ings on the smaller moths, are, as it were, placed more closely together 

 and thus obscure more completely the (generally) paler ground colour. 

 As all the larger races of the species mentioned feed on bushes, shrubs, 

 willow, &c.,and the smaller ones on low-growing plants (Vaccmmm), it 

 can readily be seen that the surroundings of these races are very 

 different, and the influence of "natural selection" would be correspond- 

 ingly large. 



In an introduction of this kind, anything like an exhaustive essay 

 would undoubtedly be out of place, but I think sufficient has been 

 written to show that the phases of variation in our British NocTU-as are 

 many and striking, that the causes of these are as yet most imperfectly 

 known and exceedingly complex in their character, and that a large 

 field is open to scientific workers in this direction as the systematic 

 classification of the material that we have continually coming to hand 

 is worked into a more definite form. 



