TIME RELATIONSHIPS OF MIMICRY 249 



on the point. Already it proves that the members of 

 the groups which converge round Limnas chrysippus, 

 Amauris echeria, and the black and white species of 

 Amauris in Natal are upon the wing together. This 

 evidence has been very kindly supplied me by Mr. Guy 

 A. K. Marshall. I have similar, but less complete, 

 evidence as regards some of the Central and South- 

 American groups. 



It will probably be conceded that the phenomena 

 generally are likely to exhibit the same relationship in 

 time which has been already proved to exist for many 

 of them. This conclusion, however, is a considerable 

 difficulty in the way of the theory of External Causes as 

 well as a further difficulty to the theory of Internal 

 Causes. As regards the latter, the time relationship is 

 an entirely unexplained coincidence ; as regards the 

 former, it is a coincidence which leaves much to be 

 explained. It is difficult enough to believe that local 

 forces could produce local resemblance ; it is a further 

 difficulty that the resemblances are contemporaneous. 

 If, as is probable, the forces are supposed to act during 

 larval life, they must include in their effects an influence 

 on the rate of growth and development, an adjustment 

 of the duration of stages delicate enough to bring the 

 various species into the phases in which the resemblance 

 is shown at similar times of the year. But such effects 

 are entirely different from those which are manifest in 

 the resemblance itself, and add a further complexity to 

 a result already shown to be so complex that the theory 

 of External Causes fails to supply an interpretation (see 

 Section 7, p. 240). 



But it has been shown in many cases, and is probably 

 true in all, that the time relationships between the species 

 which exhibit these resemblances are not confined to their 

 appearance at the same season of the year. They are 

 such that they fly together under those conditions of 

 light which render the resemblance visible to enemies. 

 When moths resemble butterflies, they are mostly species 

 which are as truly day-flying as the butterflies themselves ; 

 in other cases they are species which fly readily by day 



