ORIGIN OF SPECIES 



173 



who has studied the group has doubted for a moment 

 that the two are perfectly and originally distinct species, 

 like the hare and rabbit, for instance, or any other two 

 allied species of one and the same genus. The following 

 facts, however, led me to conclude that the one is simply 

 a modification of the other. There are, as might be 

 supposed, districts of forest intermediate in character 

 between the drier areas of Obydos, &c., and the moister 

 tracts which compose the rest of the immense river 

 valley. At two places in these intermediate districts, 

 namely, Serpa, 180 miles west of Obydos, on the same 

 side of the river, and Aveyros, on the lower Tapajos, most 

 of the individuals of these Heliconii which occurred were 

 transition forms between the two species. Already, at 

 Obydos, H. Melpomene showed some slight variation 

 amongst its individuals in the direction of H. Thelxiope, 

 but not anything nearly approaching it. It might be 

 said that these transition forms were hybrids, produced 

 by the intercrossing of two originally distinct species ; 

 but the two come in contact in several places where these 

 intermediate examples are unknown, and I never ob- 

 served them to pair with each other. Besides which, 

 many of them occur also on the coast of Guiana, where 

 H. Thelxiope has never been found. These hybrid-look- 

 ing specimens are connected together by so complete a 

 chain of gradations that it is difficult to separate them 

 even into varieties, and they are incomparably more 

 rare than the two extreme forms. They link together 

 gradually the wide interval between the two species. 

 One is driven to conclude, from these facts, that the 

 two were originally one and the same : the mode in which 

 they occur and their relative geographical positions being 

 in favour of the supposition that H. Thelxiope has been 

 derived from H. Melpomene. Both are nevertheless good 

 and true species in all the essential characters of species ; 

 for, as already observed, they do not pair together when 

 existing side by side, nor is there any appearance of 

 reversion to an original common form under the same 

 circumstances. 



In the controversy which is being waged amongst 

 Naturalists, since the publication of the Darwinian theory 

 of the origin of species, it has been rightly said that no 

 proof at present existed of the production of a physio- 

 logical species — that is, a form which will not interbreed 



