INTRODUCTION 



19 



areas of the wings, has been shown by Professor Poulton not to be due to consanguinity 

 inasmuch as the transparency is brought about in the two species by entirely different 

 means. A microscopic examination shows that in Thyridia the scales are very short, and each 

 alternate scale has been reduced to a very narrow form almost like a hair. In Ituna the 

 scales are still of normal size but are widely separated. Had the transparency been due to 

 close relationship, it is reasonable to suppose that it would have been produced by the 

 same structural formation in both species. 



Miiller continues his remarks in pointing out that both species may be assumed to be pro- 

 tected from the fact that both emit a repugnant odour ; the organ by which this is produced 

 in Thyridia megisto being a tuft of hair placed on the upperside of the hind-wing on the 

 subcostal nervure, and is present in both sexes, though not so well developed in the female, 

 the case being probably one of transmission of a male characteristic to the female sex.^ 



Taking the case as undoubtedly one of mimetic resemblance, the author proceeds to 

 consider the question as to which of the species is the original form and which the imitator. 

 In this instance the problem is not one which is easily solved, since both insects appear in 

 about equal numbers, or at least one does not predominate sufficiently to be regarded as 

 the model. Again, though Thyridia possesses a somewhat larger number of similar allies 

 than does Ituna, neither insect appears to depart sufficiently from the ' family facies ' to enable 

 it to be regarded as the imitator. Both species being apparently distasteful, there is again 

 nothing to serve as a guide to the desired distinction. The Miillerian explanation of the 

 phenomenon of resemblance between two species of protected genera depends on the assump- 

 tion that insectivorous birds, lizards, &c., do not instinctively know what is good to eat 

 and what is distasteful or injurious. That the assumption is well founded I shall endeavour 

 to show later on. Meanwhile it is desirable to quote here the words of Professor Meldola's 

 translation of Miiller' s paper describing the advantage gained by mimicry between two 

 already protected butterflies. 



' Now if two distasteful species are sufficiently alike to be mistaken for one another, 

 the experience acquired at the expense of one of them, will likewise benefit the other ; both 

 species together will only have to contribute the same number of victims which each of 

 them would have to furnish if they were different. If both species are equally common, then 

 both will derive the same benefit from their resemblance — each will save half the number 

 of victims which it has to furnish to the inexperience of its foes. But if one species is com- 

 moner than the other, then the benefit is unequally divided, and the proportional advantage 

 for each of the two species which arises from their resemblance is as the square of their 

 relative numbers. 



* For instance, let us suppose that in a given region during one summer 1,200 butterflies 

 of a distasteful species have to be destroyed before it becomes recognized as such, and that in 

 this region there exist 2,000 individuals of one [A) and 10,000 of another {B) distasteful 

 species. If they are quite different, each species will lose 1,200 individuals ; but if they 

 are deceptively alike, then this loss will be divided among them in proportion to their numbers, 

 the first [A) will lose 200, and the second [B) 1,000. The former {A) accordingly gains 1,000 

 (or 50 per cent.) of the total loss, and the latter {B) only 200 (or 2 per cent.) of this number. 

 Thus while the relative number of the two species is in the ratio of i : 5, the advantage 

 derived by those possessing the resemblance is 25 : i.^ 



^ The fact that these glands are not now con- 2 xhe precise mathematical statement of the case 



sidered to be necessarily the seat of distasteful is somewhat complicated. See Blakiston and Alex- 

 qualities does not affect the argument. ander. ' Nature,' 1883, pp. 481-2, and 1884, pp. 405-6. 



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