IX WARNING COLORATION AND MIMICRY 253 



fact, that insect-eating birds only learn by experience to 

 distinguish the edible from the inedible butterflies, and in 

 doing so necessarily sacrifice a certain number of the latter. 

 The Cjuantity of insectivorous birds in tropical America is 

 enormous ; and the number of young birds which every year 

 have to learn ^^isdom by experience, as regards the species of 

 butterflies to be caught or to be avoided, is so gi'eat that the 

 sacrifice of life of the inedible species must be considerable, 

 and, to a comparatively weak or scarce species, of vital im- 

 portance. The number thus sacrificed will be fixed by the 

 quantity of young birds, and by the number of experiences 

 requisite to cause them to avoid the inedible species for the 

 future, and not at all by the numbers of individuals of which 

 each species consists. Hence, if two species are so much 

 alike as to be mistaken for one another, the fixed numoer 

 annually sacrificed by inexperienced birds ^vill be di\aded be- 

 tween them, and both will benefit. But if the two species are 

 very unequal in numbers, the benefit will be comparatively 

 slight for the more abundant species, but very gTeat for the rare 

 one. To the latter it may make all the difterence between 

 safety and destruction. 



To give a rough numerical example. Let us suppose that 

 in a given limited district there are two species of Heliconidie, 

 one consisting of only 1000, the other of 100,000 indi^-iduals, 

 and that the quota required annually in the same district for 

 the instruction of young insectivorous birds is 500. By the 

 larger species this loss will be hardly felt ; to the smaller it 

 will mean the most dreadful persecution resulting in a 

 loss of half the total population. But, let the two species 

 become superficially alike, so that the birds see no difl"erence 

 between them. The quota of 500 ^\all now be taken from a 

 combined population of 101,000 butterflies, and if propor- 

 tionate numbers of each sufl*er, then the weak species will 

 only lose five individuals instead of 500 as it did before. 

 Xow we know that the difi'erent species of Heliconidae are 

 not equally abundant, some being quite rare ; so that the 

 benefit to be derived in these latter cases would be very im- 

 portant. A slight inferiority in rapidity of flight or in powers 

 of eluding attack might also be a cause of danger to an in- 

 edible species of scanty numbers, and in this case too the being 



