282 Studies in the Theory of Descent. 



genus Sphinx. In some specimens of Smerinthus 

 Tilite the spots coalesce so as to form an irregular 

 coloured edge to the oblique stripes. In S. Populi 

 they occur in many individuals, but remain always 

 in the spot stage ; whilst S. Ocellatus is but seldom, 

 and S. Quercus appears never to be spotted. 



The spots both of S. Tilice and Populi certainly 

 do not show themselves exclusively in the fifth 

 (last) stage, but also in the fourth, and sometimes 

 in Populi even as early as the third stage, from 

 which we might be disposed to conclude that the 

 new character did not first appear in the last stage. 

 But the majority of the spotted individuals first 

 acquire their spots in the fifth stage, and only a 

 minority in the fourth ; so that their occasional 

 earlier appearance must be ascribed to the back- 

 ward transference of a character acquired in the 

 fifth stage. Moreover, the fourth and fifth stages 

 of the caterpillars are closely analogous both in size, 

 mode of life, and marking, and are therefore ana- 

 logous with reference to the environment, so that it 

 is to be expected that new characters, when depend- 

 ing on adaptation, would be rapidly transferred 

 from the fifth stage to the fourth. 3 We should 



8 [If the reddish-brown spots on the larva of S. Populi have 

 the protective function assigned to them by Mr. Peter Cameron 

 (Trans. Ent. Soc. 1880, p. 69), it can be readily understood 

 that they would be of service to the insect in the fourth 

 stage, and the backward transference of this character might 

 thus be accelerated by natural selection, in accordance with 

 the above principles. (See, als.o, note 30 , p. 241.) R.M.] 



