number of cases, increasing almost every day, it is found that utility 

 either to the individual or the species is at the bottom of the charac- 

 ters which come under our observation, whether of aspect, structure 

 or behaviour. The Darwinian view of organic nature is in fact 

 strictly teleological, a term the use of which has sometimes been 

 considered, I venture to think undeservedly, as involving a kind of 

 reproach. Now is there, it may be asked, any reason to suppose 

 that the phenomenon of seasonal dimorphism is of any benefit to 

 the species which exhibit it ? It would be difficult to give an 

 answer that would apply to all the know'n cases. We should have, 

 I think, to admit that the benefit, if any, is not always obvious. 

 We can sometimes form conjectures of a greater or less degree of 

 probability ; something may be due to the past history of the species 

 if it could be unravelled, and still more perhaps to what is called 

 " correlation " with a really useful feature. But there certainly are 

 cases in which we can assign a useful purpose to the seasonal 

 change with a very high degree of probability indeed. Such a case 

 is to be found in the genus Precis that we have just been 

 considering. 



Looking at the undersides of several species of Precis, we can see 

 at once that they strongly resemble dead and withered leaves. The 

 midrib of the leaf is represented by a stripe running across both 

 fore- and hindwing, from the tip of the former to the posterior 

 angle of the latter. This angle is often prolonged to look like a 

 stalk, and the surface of the wings is apt to be mottled in a manner 

 that suggests spots of decay or growths of microscopic fungi. Ex- 

 amining the matter a little more closely, we find that in many cases 

 it is only the dry-season phase of the species that puts on the dead- 

 leaf appearance, the wet-season phase of the same species showing 

 no such resemblance at all. (See P. simia and P. antUope.) In 

 other species both seasonal forms are like dead leaves beneath ; but 

 when this is the case, the resemblance is greater in the dry season 

 than it is in the wet. Now when an insect goes out of its w^ay, so 

 to speak, in order to look like a leaf, a stick, or a bundle of dry 

 grass, we may conclude with very great probability that there is a 

 meaning in this departure, and that the meaning is the necessity of 

 concealment, either for the purpose of approaching its prey unob- 

 served, or of itself escaping the attention of enemies who desire to 

 prey on it. We may therefore put down these dead-leaf appearances 

 as instances of what Professor Poulton has called cryptic coloration, 

 that is to say, coloration whose object is concealment. But now the 

 question arises, why should this kind of protection be so much 

 better developed in the dry season than in the wet, and why in some 

 cases should it not be found in the wet-season forms at all ? To 

 this question a reasonable answer can be given. 



It has been pointed out by Professor Poulton that insect-eating 

 animals of all sorts jn the tropics find it much harder to get their 

 food in the dry season than in the wet. Many insects hide them- 



