May 25, 1882] 



NATURE 



87 



of each distinct species. But if two species, both equally 

 distasteful, closely resemble each other, then the number 

 of individuals sacrificed is divided between them in the 

 proportion of the square of their respective numbers ; so 

 that if one species (a) is twice as numerous as the other (i), 

 then b will only lose one-fourth as many individuals as it 

 would do if it were quite unlike a ; and if it is only 

 one tenth as numerous then it will benefit in the propor- 

 tion of 100 to 1. It is an undoubted fact that the species 

 of protected butterflies, like those of other group-, differ 

 greatly in abundance of individuals, some being very rare 

 while others are among the commonest of all butterflies. 

 The proportion of ico to 1, therefore, is far below the 

 amount of benefit an uncommon species might derive by 

 resembling a common one. The benefit to be derived is 

 thus clear, if the protected species are subject to the 

 danger of attacks by young birds before they learn that 

 such species are uneatable. I agree with Dr. Miiller that 

 they are exposed to this danger ; and when we con^der 

 the great number and variety of insectivorous birds in 

 South America the danger must be considerable, and 

 quite sufficient to render it important for a numerically 

 weak species to reduce it to a minimum, although to a 

 species abounding in individuals it may be of lit' le im- 

 portance. It has been suggested that young birds have 

 an hereditary instinct enabling them to distinguish uneat- 

 able butterflies antecedent to experience ; but this seems 

 in the highest degree improbable. It has no doubt been 

 shown by Mr. Darwin that monhe\s in captivity have a 

 dread of snakes, and Mr. Jenner YYeir believes that birds 

 have an instinctive knowledge of uneatable caterpillars. 

 But even admitting that in these two cases there is an 

 instinctive hereditary aversion, it does not follow that the 

 same will occur with regard to protected butterflies. 

 Snakes form one well-marked group, and it is not alleged 

 that monkeys distinguish between poisonous and harmless 

 snakes : and caterpillars can also be readily divided into 

 the two classes of edible and inedible by their green or 

 brown (protective) colours on the one hand, and their 

 gaudy or con-picuous colouration or hiiry bodies on the 

 other. But the protected butterfles have no such general 

 mark of inedibility. Their colours and forms vary greatly, 

 and cannot as a group be readily differentiated from those 

 of other butterflies ; and it is not to be accepted without 

 actual proof that a young bird knows instinctively every 

 Helicono d or Danacoid butterfly in its district, as well 

 as the protected Papilios and moths, almost infinitely 

 varied as they are in co'our and marking, among the 

 equally numerous and equally varied butterflies of other 

 groups It therefore seems clear to me that we have 

 here a vera causa for the acquisition of true protective 

 mimicry by the less abundant species of inedible 

 butterflies. 



There is however yet anoiher cause which may have 

 led to mimicry in these cases, and one which does not 

 appear to have been discus=ed by Dr. Muller. The fact 

 that the majority of butterflies are edible and are actually 

 eaten by birds and other insectivorous creatures, while a 

 considerable minority are distasteful and are thus pro- 

 tected, renders it pretty certain, a priori, that there exist 

 many degrees of di-tastefulness. Certain species appear 

 to be rejected by all insectivorous creatures, while some, 

 though not eaten by bird-, may be devoured by lizards, 

 dragon-flics, or spiders. Some, too, may be eaten by 

 some birds and rejected by others, and no ornithologist 

 will think it strange or improbable that a trogon should 

 have somewhat different tastes from a tyrant-shrike or a 

 swallow. Again, in some species the distastefulrcss may 

 extend to all the stages of egg, larva, pupa, and perfect 

 insect, while in others it may be confined to one or more 

 of the-e stages ; or special dangers may exist for one 

 species which are ab-ent in the case of another. Hut it 

 is evident, tint, if these differences exist, it will be advan- 

 tageous for the less protected to mimic the more com- 



pletely protected species, and the fact of the affinity 

 between the different genera, with perhaps some tendency 

 to revert to a common style of colouration or marking, 

 will afford facilities for the development of this class of 

 mimicry even greater than occur in the case of the dis- 

 tinct and often remote families of completely unprotected 

 butterflies. We need not, therefore, be surprised to find 

 whole series of species of distinct genera of Heliconoid 

 butterflies apparently mimicking each other ; for such 

 mimicry is antecedently probable on account of the 

 greater need of protection of some of these species than 

 others, arising either from some species being less dis- 

 tasteful to certain enemies, or less numerous, and therefore 

 likely to suffer to a serious extent by the attacks of inex- 

 perienced birds. When these two conditions are com- 

 bined, as they often would be, we bave everything necessary 

 for the production of mimicry. 



The explanation now given, so far as it refers to the 

 various degrees of protection, may be extended to explain 

 those cases in which various groups of Nymphalidae or 

 other families appear to mimic each other; such as Cata- 

 gramma, Callithea, and Agrias in one series, and Apatura 

 with Heterochroa in another. In my " Tropical Nature" 

 (p 2 57) I have remarked — " Here, again, neither genus 

 is protected, and the similarity must be due to unknown 

 local causes'' ; but this is mote than we know, and I now 

 think that some of these groups — perhaps Catagramma 

 and Heterochroa — are panially protected, and the advan- 

 tage of sharing in this partial protection has led species 

 of altogether unprotected and much persecuted groups to 

 gain some protection by mimicking them, whenever their 

 general form, habits, \ and style of coloration offered a 

 suitable groundwork for variation to act upon. 



If these views are correct we shall have the satisfaction 

 of knowing that all cases of mimicry are explicable by 

 one general principle ; and it seems strange to me now 

 that I should not have seen how readily the principle is 

 applicable to these abnormal cases. The merit of the 

 discovery is however wholly due to Dr. Fritz Muller; and 

 it is to be hoped that he will complete his work by obtain- 

 ing, if possible, evidence of its correctness. The chief 

 thing required is an experimental proof of various degrees 

 of inedibility in butterflies, during the different stages ol 

 their life-history ; and also some observations as to the 

 comparative abundance of the species of protected butter- 

 flies which mimic each other. If to this can be added 

 the proof that such groups as Catagramma, which seem 

 to be the objects ot mimicry, are partially protected by 

 inedibility, the chief remaining difficulty in the application 

 of the theory of natural selection to all known cases of 

 protective imitation will have been cleared up. 



Alfred R. Wallace 



NOTES 



In reference to the Darwin Mem jrial, to which we referred 

 last w eek, the honorary secretaries have issued a circular asking 

 for contril ution> to the fund. In this memorial it is ^tited that 

 th. ugh the works of Charles Darwin are his hest and most 

 enduring memorial, it is felt by his many friends and admirers 

 that th. se should not lie the only one. They are desirous of 

 banding down to poslerity the likeness of a man who has done 

 so much for [he advancement of natural knowledge. They wish 

 also to estai loh a fund associated with his name, the proceeds 

 of which will be devoted to the furtherance of biological sc ier.ee 

 A committee has accordingly teen formed, of which Mr. T. G. 

 Bouncy and Mr. P. Edward Dove are the nan. secretaries. The 

 committee is one of the highest influence, compri-ing the leading 

 foreign n inisters, the two Archj I the best-known 



names in all ran 1 s and 



Johann Carl Friedrich Zoli.ner, whose death we 

 recently announced, was I orn at Berlin on November 8, 1834. 



