Page:The naturalist on the River Amazons 1863 v1.djvu/288

 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 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 throughout all that area; some slight variations only occurring at the extreme points of it. If local conditions