72 THE EVOLUTION THEORY 



is in its full-grown stage green, like the wild convolvulus on which it 

 lives, or brown like the ground on which its food-plant grows. It thus 

 shows a double adaptation, each of which is capable of protecting it to a 

 certain extent, and we might think to the sarnie extent. But that is 

 not so, the brown colouring is a more effective protection than the 

 green, as we may learn from two facts. In the first place, the four 

 young stages of the caterpillar are green, and it only becomes brown 

 in the last stage, though sometimes even then it remains green. This 

 shows that the brown is a relatively modern adaptation, and it could 

 not have arisen had it not been better than the original green. In the 

 second place, the green-coloured caterpillars of the convolvulus hawk- 

 moth are nowadays much less numerous than the brown ones, and this 

 implies that the latter survive oftener in the struggle for existence. 

 We have here an interesting case of an easily recognizable process of 

 selection still going on between the old green and the newer brown 

 variety. 



It is hardly necessary to ask why the brown colour should in this 

 case be a better protection than the green, for it is obvious that such 

 a large green body as that of the full-grown convolvulus-caterpillar 

 would be but badly concealed among the little leaves of the con- 

 volvulus plant in spite of its green colour; while the brown caterpillar, 

 on the brown soil, with its pebbles, hollows, and irregular shadows, is 

 excellently protected, especially if it passes the day concealed in the 

 ground, as is actually the case. 



Our view is materially strengthened by the fact that the same 

 phenomenon of double colouring occurs in several allied species of 

 Sphingidse, but in a manner which shows us that we have to do with 

 a similar process of transformation, only at a more advanced stage. 

 The caterpillar of Chcerocampa elpenor (Fig. 4) shows the same state 

 of things as that of the convolvulus hawk-moth ; it is brown or green, 

 and the green form is the less common. But in the two other European 

 species of Chcerocampa the full-grown caterpillar is always brown, 

 and indeed it becomes brown in the fourth stage, instead of, like 

 Chcerocampa elpenor, only in the fifth and last. Another indigenous 

 sphingid species, Deileplvila vespeiiilio, only remains green during the 

 first two stages, and assumes in the third stage the grey-brown colour 

 which it afterwards retains. The dark colour has obviously prevailed 

 among the full-grown caterpillars for a considerable length of time, 

 for it is in this, the largest and most conspicuous stage, that the 

 change of colour must have been most necessary, and consequently 

 the process of selection must have begun in it, and only after the more 

 protective brown became general would it have extended to the next 



