OF THE AMAZON VALLEY. 
50] 
those which supply the most decisive results are Media nith JPolymnia, Ithomia Orolina 
aud IHinissa, Ceratinia Ninonia, and the Zycorece. 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 locality without intercrossing, as takes place in Ithom w 
Blinissa and its allies, and probably in Medianitis Nescea and Lusimnia. 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 m Heliconius 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 HeUconida 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 Heliconius (H. Rhea, Clytia, Ricini, Vesta, Thelxiope, Ant iodic, 
&c.) are unchanged over the whole of the wide country which includes the areas of 
several successive local races of many Ithomia and Napeogeties *. 
The process of the creation of a new species I believe to be accelerated in the Itho- 
mia 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. 
O"-— ■"-«■— O 
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. \Ye see here the manufacture, as it were, in 
process. The species is widely disseminated and variable. The external conditions in 
certain localities 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. Hga'eusis) 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 aU 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 with 
* These are Ithomia Flora, an inhabitant of the whole Lower Amazon region (from the Atlantic to the Rio Negro), 
which is represented by J. Hippodamia in Cayenne, and 7. Onega, Illinissa, Gunilla, Priscilla, Eerdina, m different 
areas on the Upper Amazon ; Napeogenes Cyrianassa, which becomes N. adelphe on the banks of the Cupan (Tapajos), 
and N. Tunantina on the north bank of the Upper Amazons ; N. Inaehia, which is changed to .V. sidphurina at 
Bahia, and to N. Ercilla, N. Corena 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. 
