262 THE LOWER AMAZONS. Chap. VL 



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, there- 

 fore, 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 cUmate, causing delayed or quickened growth, 

 — have great effect on insects, acting on their adoles- 

 cent states, and so by correlation of growth on the 

 shape and colours of the adult forms.* But there is 

 no proof that a complete local variety or race has been 

 produced wholly by this means, modifications acquired 

 by individuals not being generally transmissible to 

 offspring. The examination of these races or closely 

 allied species of Heliconii, with reference to their geo- 

 graphical distribution, throws light also on this subject. 

 Thus Heliconius Thelxiope is disseminated over a dis- 

 trict 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 through- 

 out all that area ; some slight variations only occur- 

 ring at the extreme points of it. If local conditions 



* M. Bellier de la CliaA'ignerie, in the "Annales de la Society Ento- 

 mologique 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 insects are producible 

 by this means. 



