61 



to be very similar, but each species has apparently replied 

 differently to the demands made upon it, so that the larvas 

 differ, even in the first instar, although then very much 

 alike ; and in the last instar we all know they differ widely 

 in appearance. The environment of the imagines of both 

 species has been, at least, so similar that they scarcely at all 

 differ in appearance. There can be little doubt, I imagine, 

 that the same outside forces have brought about the singular 

 forms now assumed by the larvae of the Cerurids, the dorsal 

 hump and the long tails with flagelli. Dicranura vimila res- 

 ponded so that the highest point of the hump rises on the 

 metathorax, and the flagellum is red ; Ccrula bifida and 

 C. furcula replied by making the mesothorax the highest 

 point, and their flagella are black. 



Apamea ophiogvamnia is an extremely constant species. The 

 imago varies scarcely at all. On the other hand Apamea didyma 

 is one of the most variable of Noctuse that is known. We may 

 take hundreds at sugar in the same field and yet not get 

 two exactly alike. Where we take the former species we shall 

 find the latter as well, and they are evidently very closely 

 allied, for there is a form of A. didyma which bears a very 

 strong resemblance to A. ophiogravima. May it not be, in 

 these two cases, that the inner forces are not quite the same 

 in kind. The inner hereditary forces in A. ophiogramma may 

 be very strong and conservative, tending to keep the insect 

 within very strict lines, while the innate tendencies of A. 

 didyma may not only allow, but may even induce, great 

 variation. A similar but not so marked case may be cited 

 with respect to Xylophasia monoglypha and A", lithoxylea, the 

 former being a variable species and the latter a very constant 

 one. 



I suppose we must presume that all variation is due, in 

 the first instance, to the influence of the external factors 

 or outside forces, but it is not difficult to perceive that, if 

 once variation arise, then the tendency to variation may 

 become one of the inner forces or hereditary factors. Mr. Tutt 

 mentions that in the wet season of 1888, many species of 

 Lepidoptera at Deal assumed a darker colour than usual. 

 He instances Xylophasia monoglypha, which at Deal is usually 

 of the pale variegated form with an occasional ab. obsctira, a 

 suffused brown form ; and he states that in that year this 

 species was in greater abundance than usual, but almost all 

 were ab. obsctira (" Ent. Rec," vol. i, p. 121). Taking this 

 for our case, let us, for the sake of illustration, imagine a 

 range of hills, one side facing the drier east and the other the 



