330 THE PLACE OF MIMICRY 



can be made clear if we are thus led to realize how 

 it is that a higher degree of resemblance is more 

 advantageous than a lower degree. For this reason 

 I have set forth below, by means of a hypothetical case, 

 the principles which, in my opinion, have led to the 

 gradual growth of Synaposematic likeness. 



If we suppose (i) that two species of butterfly, A and 

 B, living in the same locality are equally distasteful to 

 birds, and that young birds have to learn their qualities 

 by the test of experience before avoiding them for the 

 future ; (2) that during this education each young 

 insectivorous bird destroys or prevents the reproduction 

 of one example of each pattern ; (3) that the numbers of 

 A and B are equal, but that half the individuals of B 

 possess a pattern A ', indistinguishable from A, and the 

 other half, a pattern B' sufficiently distinct to require 

 learning by a separate set of tasting experiments, it 

 follows that, if the inexperienced insectivorous birds of 

 a given area be estimated at 20,000 and the two butter- 

 flies at 1,000,000 each, then 20,000 losses will be suffered 

 by the 1,500,000 individuals of pattern A + A' and an 

 equal number by the 500,000 of pattern B '. In other 

 words, pattern A' will lose if / and B' 4/ r . 



In the vast majority of cases, however, the two species 

 are not equal in numbers, but those of the mimicking 

 species very much smaller. If, for the sake of illustration, 

 we suppose the numbers of A to be 1,750,000 and those 

 of B 250,000 divided as before into A' and B ', it follows 

 that A + A' would lose 20,000 out of 1,875,000 or a little 

 over i I (about 1-07), and & 20,000 out of 125,000 or 

 i6/ o . It is thus possible at once to see why, when the 

 numbers become very disproportionate, the only appre- 

 ciable approach is from the side with the smaller number 

 of individuals. 



The advantages of Diaposematic or Reciprocal Resem- 

 blance may be illustrated in the same manner. Taking 

 the numbers of the first illustration, let us suppose that 

 half the individuals of A t viz. A*, advance to meet an 

 equal advance of half the individuals of B, viz. B* t so 

 that the two form a single pattern A* + B* separate 



