ORIGIN OF SPECIES 



175 



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 in- 

 dividuals 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 in- 

 sects, acting on their adolescent states, and so by cor- 

 relation 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 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 through- 

 out all that area ; some slight variations only occurring 

 at the extreme points of it. If local conditions acting 

 directly on individuals had originally produced this race 

 or species, they certainly would have caused much modi- 

 fication of it in different parts of this region ; for the 

 upper Amazons country differs greatly from the district 

 near the Atlantic in climate, sequence of seasons, soil, 

 forest clothing, periodical inundations, and so forth. 

 These differences moreover graduate away, so that the 

 species is subjected to a great diversity of physical con- 

 ditions from locality to locality, and ought in consequence 



^ M. Bellier de la Chavignerie, in the Annales de la Sociite 

 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 insects are producible by this means. 



