OF THE AMAZON VALLEY. 513 



(55° W. long.) the resemblance is perfect (PL LV. fig. 1) ; and this is the only form of 

 the Leptalis known in the locality. The varieties figured PI. LVI. figs. 1, 2, 3, show 

 diflFerent degrees of resemblance to Ithoniia Chrysodonia (fig. 3 a) ; these, therefore, 

 exhibit the selection in process. Thus, althoiigh we are unable to watch the process of 

 formation of a new race as it occxu's in time, we can see it, as it were, at one glance, by 

 tracing the changes a species is simultaneously undergoing in different parts of the area 

 of its distribution. 



The fact of one of the forms of Leptalis Theonoe, namely L. Lysinoe, mimicking at Ega, 

 not an Ithomia, but a flourishing species of another quite distinct family {StalacUis 

 Duvalii), shows that the object of the mimetic tendencies of the species is simply dis- 

 guise, and that, the simple individual differences in that locality being originally in the 

 direction, not of an Ithomia, but of another object equally well answering the purpose, 

 selection operated in the direction of that other object. This point is well illustrated 

 by the species of a small group of Longicorn Beetles already cited, some of which mimic 

 a piece of bark, and others insects of another family — and by hunting Spiders, many of 

 which wear the form of insects, and many that of inanimate objects amongst which 

 they seek their prey. 



When the persecution of a variable local form of our Leptalis is close or long con- 

 tinued, the indeterminate variations naturally become extinct ; nothing then remains in 

 that locality but the one exact counterfeit, whose exactness, it must be added, is hence- 

 forward kept up to the mark by the insect pairing necessarily with its exact counterpart, 

 or breeding in and in. This is the condition of Leptalis Theonoe (PI. LV. fig. 1) in its 

 district ; and it is the condition of all those numerous species of different orders which 

 now appear fixed and distinct. When (as happens at St. Paulo, where a greater abund- 

 ance of individuals and species, both of Ithomia and Leptalis, exists than in the locality 

 of the last-named) many species have been in course of formation out of the varieties of 

 one only, occasional intercrossing may have taken place ; this would retard the process 

 of segregation of the species, and, in fact, aid in producing the state of things (varieties 

 and half-formed species) which I have already described as there existing. 



In what way our Leptalis originally acquired the general form and colours of Ithomice 

 I must leave undiscussed. We may conclude (if we are to reason at all from existing 

 facts) that, as the antecedent forms of our races of Leptalis which are still undergoing 

 change were themselves similar to Ithomice, the form has been inherited through a long 

 line of ancestors, which have been more or less subjected to similar conditions. The 

 instance of one of our forms leaving the Ithomice to mimic a species of another family 

 may show us how a new line of mimetic analogy and gradual modification may have 

 been originally opened. 



Such, I conceive, is the only way in which the origin of mimetic species can be ex- 

 plained. I believe the case offers a most beautiful proof of the truth of the theory of 

 natural selection. It also shows that a new adaptation, or the formation of a new species, 

 is not effected by great and sudden change, but by numerous small steps of natural 

 variation and selection. Some of the mutual resemblances of the Heliconidce already 

 mentioned seem not to be due to the adaptation of the one to the other, but rather, as they 



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