403 



noticed the fact that some of the larger water-beetles belonging to 

 the genus Dyticus " have females of two forms, the most common 

 having the elytra deeply sulcate, the rarer smooth as in the males." 

 Mr. Wallace remarks that it is in " the female sex alone that these 

 distinct forms generally occur ;" and in his work on " Natural 

 Selection" he has largely illustrated this fact by reference to the 

 Papilionidse of the Malay Archipelago. He has even suggested the 

 way in which " dimorphism may be produced." Taking the case 

 of a particular species of butterfly found in most of those islands, 

 and in which there are two extreme forms of the female, one 

 occurring in Sumatra, the other in Java, Borneo, and Timor, he goes 

 on to remark that an allied species is found in the Phillipine 

 Islands with the same two extreme forms of females, along with a 

 number of intermediate varieties. K then we suppose " the 

 extreme Phillippine forms to be better suited to their conditions of 

 existence than the intermediate connecting links, the latter will 

 gradually die out, leaving two distinct forms of the same insect, 

 each adapted to some special conditions. As these conditions are 

 sure to vary in different districts, it will often happen, as in 

 Sumatra and Java, that the one form will predominate in the one 

 island, the other in the adjacent one." 



The same reasoning might be found to bear upon cases of di- 

 morphism in the British Isles, or in different localities in England, 

 and serve perhaps to explain the marked variations of form and 

 colouring that occur in certain species of our own Lepidoptera, the 

 varieties being sometimes widely apart, and confined to very limited 

 districts. 



A remarkable case of dimorphism in a British moth (Acronycta 

 leporinaj was not long since exhibited to the Entomological Society, 

 " the right hand wijags being coloured and marked as in the variety 

 known as hradyporina (at one time considered a distinct species), 

 whereas those of the left hand were entirely typical of leporina. 

 The body also partook of the two forms, being divided longi- 

 tudinally into two tints." * This circumstance is of much interest, 

 as not only speaking to the fact of the existence of dimorphism, but 

 as presenting us with a clear case in which two varieties of a moth, 

 * Ent. Trans., 1872. Proceed., p. x. 



