OF THE AMAZON VALLEY. 501 



those which supply the most decisive results are Mechanitis Fohjmnia, ItJiomia Orolina 

 and HUnissa, Ceratinia Ninonia, and the Lycorece. The varieties of these present all the 

 different grades between simple individual differences and well-marked local varieties or 

 races, which latter cannot be distinguished from true species, when two or more of them 

 are found coexisting in the same locahty without intercrossing, as takes place in ItJwmia 

 Illiiiissa and its allies, and probably in 3£echcmitis Nescea and Lysminia. A striking 

 case of the production of a local variety now spread over a wide area, and undistin- 

 guishable from a true species, is afforded in ITeUcomus Thelxiope, to the details of which, 

 given in the systematic part of this memoir, I must refer the reader. 



These species, when carefully studied, seem to me conclusively to show that many of 

 the now distinct species of HeUconidce have arisen from local varieties, segregated from 

 the variations of preexisting widely disseminated species ; for these distinct forms or 

 species do not essentially differ from the undoubted varieties of the species cited. The 

 genera show different degrees of susceptibility of change under altered local conditions. 

 Thus, many species of Helicouius {H. Bhea, Clytia, Bicini, Vesta, Thelxiope, Antiocha, 

 &c.) are unchanged over the whole of the wide country which includes the areas of 

 several successive local races of many Ithomice and Napeogenes*. 



The process of the creation of a new species I believe to be accelerated in the Itho- 

 mics and allied genera by the strong tendency of the insects, when pairing, to select 

 none but their exact counterparts : this also enables a number of very closely allied ones 

 to exist together, or the representative forms to live side by side on the confines of their 

 areas, without amalgamating. 



The course followed by Nature in the formation of these numerous local species, I 

 think, is clearly exhibited in Mechanitis Folymnia, to the details of which, given in its 

 place, I must beg the reader to refer. We see here the manufacture, as it were, in 

 process. The species is widely disseminated and variable. The external conditions in 

 certain locaUties are more favourable to one or more of the varieties there existing than 

 to the others ; those favoured ones, therefore, prevail over the others. We find, in this 

 most instructive case, all the stages of the process, from the commencement of the forma- 

 tion of a local variety (var. Egaensis) to the perfect segregation of one (var. Lysimnia, 

 considered by all authors as a true species). In this species, most of the local varieties 

 are connected with their parent form by individuals exhibiting all the shades of varia- 

 tion ; and it is on this account only that we know them to be varieties. In the species 

 allied to Ithomia Flora, the forms are in a complete state of segregation (with the excep- 

 tion of I. Illinissa, which throws light on the rest), and therefore they are considered as 

 species ; they are, in fact, perfectly good species, like all other forms considered as such 

 in natural history. It is only by the study of variable species that we can obtain a clue 

 to the explanation of the rest. But such species must be studied in nature, and mth 



* These axe Ithomia Flora, an inhabitant of the whole Lower Amazon region (from the Atlantic to the Rio Negro), 

 which is represented by I. Hippodamia in Cayenne, and I. Onega, Illinissa, Gimilla, Priscilla, Ilerdina, in different 

 areas on the Upper Amazon ; Napeoyenes Cyrianassa, which becomes N. adelphe on the banks of the Cupari (Tapajos), 

 and N. Tunantina on the north bank of the Upper Amazons ; N. Inachia, which is changed to N. sulplmrina at 

 Bahia, and to N. Ercilla, N. Coi-ena and N. Pharo in different areas on the Upper Amazon. Other species might 

 be added in confirmation. Most of the species of Heliconius quoted are found unchanged over the collective areas 

 of all these forms of Ithomia and Napeogenes. 



