TRUE MIMICRY 109 



of an immune model, or in the non-appearance of suitable mimetic 

 variants, or in other conditions ; but we know that without mimicry 

 the species holds its own against its enemies. But if, in Abyssinia, 

 a female of this butterfly exhibited variations which would make her 

 resemble, in any degree, the unpalatable Danias chrysippus, these 

 mimetic variants would be less decimated than the original form of 

 female, and would, therefore, gain stability, and gradually increase 

 both in mimetic resemblance and in the number of individuals. But 

 is this any reason why the original form of the female should 

 diminish in numbers 1 In itself, certainly not ; the red mimetic 

 females could increase in number without causing any decrease of the 

 yellow ones, for the red are in no way in conflict with the yellow, 

 and we must not think of the number of individuals as so fixed for 

 each species that it cannot increase. On the contrary, it must 

 increase, as soon as the conditions of existence are permanently 

 improved, and this happens, in this case, through the mimetic protec- 

 tion of the red female. We can thus easily understand how mimetic 

 and non-mimetic females can live side by side in Abyssinia. 



In all the rest of Africa, however, there are onl}^ mimetic females 

 of Papillo merope, and none of the colour of the male ; these last, 

 therefore, have been crowded out by the mimetic form, not actively, 

 but through the more frequent survival of the mimetic form, so that 

 those like the male became gradually rarer, and finally died out — 

 that is, ceased to occur. The matter is not so simple as it seems, and 

 we shall best understand it hy thinking of the dimorphism of the 

 caterpillars of our hawk-moths, which we discussed before, in which 

 the green form in the full-grown caterpillar is less well protected than 

 the brown. In many species the brown form has crowded out the 

 green, in others brown and green occur side by side, but the green is 

 less abundant, and in some species very rare. This must be regarded 

 as the simple result of the circumstance that a higher percentage of 

 the green than of the brown caterpillars fall victims to enemies, and 

 thus, in the course of generations, the green form becomes slowly 

 but steadily rarer. This will be the case even if the newer and 

 better adaptation raises the number of individuals (the 'normal 

 number') in the species, for this increase must always be a limited 

 one, even if it be very great, which is hardly likely in this case. For 

 the normal number is not determined by the mortality at one stage, 

 but by that at all the stages of life taken together. Thus a normal 

 number always persists, notwithstanding the improved conditions for 

 the species, and, on this assumption, the form under less favourable con- 

 ditions cannot permanently hold its own with that under better con- 



