IL] CHANGES OF COLOUR. 57 



when the insect is somewhat larger, the longitudinal 

 lines are a great advantage, while subsequently diagonal 

 ones become even more important. 



The next point is the colour of the mature cater- 

 pillars. We have seen that some are green, and others 

 brown. The green ones are obviously merely those 

 which have retained their original colour. Now for 

 the brown colour. This probably makes the caterpillar 

 even more conspicuous among the green leaves than 

 would otherwise be the case. Let us see, then, 

 whether the habits of the insect will throw any 

 light upon the riddle. What would you do if you 

 were a big caterpillar ? Why, like most other de- 

 fenceless creatures, you would feed by night, and lie 

 concealed by day. So do these caterpillars. When 

 the morning light comes, they creep down the stem 

 of the food-plant, and lie concealed among the thick 

 herbage, and dry sticks and leaves, near the ground, 

 and it is obvious that under such circumstances the 

 brown colour really becomes a protection. It might 

 indeed be argued that the caterpillars, having become 

 brown, concealed themselves on the ground ; and that 

 we were in fact reversing the state of things. But this 

 is not so ; because, while we may say, as a general 

 rule, that (with some exceptions due to obvious causes) 

 large caterpillars feed by night and lie concealed by 

 day, it is by no means always the case that they are 

 brown ; some of them still retaining the green colour. 

 We may then conclude that the habit of concealing 

 themselves by day came first, and that the brown 

 colour is a later adaptation. It is, moreover, interest- 

 ing to note, that while the caterpillars which live on 



