ON THE VARIATIOX OF SPECIES. 89 



tracts which compose the rest of the immense river valley. 

 At two places in these intermediate districts, namely, Serpa, 

 180 miles w^est of Obydos, on the same side of the river, and 

 Aveyros, on the lower Tapajos, most of the individuals of 

 these Jleliconii which occurred were transition forms be- 

 tween the two species. Already, at Obydos, //. Melpomene 

 showed some slight variation amongst its individuals in the 

 direction of II. Thelxiope, but not anything nearly ap- 

 proaching it. It might be said that tliese 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 have never observed 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-looking specimens are connected together 

 by so complete a chain of gradations, that it is difficult to 

 separate them even into varieties, and they are incompa- 

 rably 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 II. Thelxiope has been de- 

 rived from H. Melpomene. Both are nevertheless good and 

 true species in all the essential chai-acters 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 beins^ waofed amono-st na- 

 turahsts, since the publication of the Darwinian theory of 

 species, it has been rightly said, that no proof at present 

 existed of the production of a physiological species, — that is, 



