Cuap. VI. FCRMATION OF LOCAL VARIETIES. 135 
few has it happened that the species which clearly appears to be the 
parent coexists with, one that has been evidently derived from it. 
Generally the supposed parent also seems to have been modified, and 
then the demonstration is not so clear, for some of the links in the 
chain of variation are wanting. The process of origination of a species 
in nature, as it takes place successively, must be ever perhaps beyond 
man’s power to trace, on account of the great lapse of time it requires. 
But we can obtain a fair view of it by tracing a variable and far-spreading 
species over the wide area of its present distribution; and a long 
observation of such will lead to the conclusion that new species in all 
cases must have arisen out of variable and widely-disseminated forms. 
It sometimes happens, as in the present instance, that we find in one 
locality a species under a certain form which is constant to all the 
individuals concerned; in another exhibiting numerous varieties ; and 
in a third presenting itself as a constant form, quite distinct from the 
one we set out with. If we meet with any two of these modifications 
living side by side, and maintaining their distinctive characters under 
such circumstances, the proof of the natural origination of a species is 
complete ; it could not be much more so were we able to watch the 
process step by step. It might be objected that the difference between 
our two species is but slight, and that by classing them as varieties 
nothing further would be proved by them. But the differences between 
them are such as obtain between allied species generally. Large genera 
are composed, in great part, of such species; and it is interesting to 
show how the great and beautiful diversity within a large genus is 
brought about by the working of laws within our comprehension. 
A few remarks on the way races are produced will be here in place. 
Naturalists have been generally inclined to attribute the formation of 
local varieties or races of a species to the direct action of physical 
conditions on individuals belonging to it which have migrated into new 
localities. It might be said, therefore, that our Heliconius Thelxiope 
of the moist forests has resulted from such operation of the local 
‘conditions on H. Melpomene, especially as intermediate varieties are 
found in districts of intermediate character and position. It is true 
that external agencies—such as food and climate, causing delayed 
or quickened growth,—have great effect on insects, acting on their 
adolescent states, and so by correlation of growth on the shape and 
colour of the adult forms.* But there is no proof that a complete 
local variety or race has been produced wholly by this means, modifi- 
cations acquired by individuals not being generally transmissible to 
offspring. The examination of these races or closely allied species of 
Heliconii, with reference to their geographical distribution, throws light 
also on this subject. Thus, Heliconius Thelxiope is disseminated over 
a district 2000 miles in length from east to west, from the mouth of the 
Amazons to the eastern slopes of the Andes, but shows no remarkable 
modification throughout all that area; some slight variations only 
* M. Bellier de la Chavignerie, in the Aznales de la Société Entomologique de 
France, 1858, p. 299, relates experiments on the effect of retardation of the pupa 
development through exposure to unusual cold, showing that striking varieties of the 
adult insect are producible by this means. 
