Letters, Announcements, i^c. 227 



into the other through the representatives of the species 

 occupying the area connecting the two above-mentioned loca- 

 lities^ in other words, a gradually increasing variation, in 

 passing southward, from the Massachusetts form toward the 

 extreme Floridan phase. 



If this were an isolated case, its significance in the present 

 connection would be of less moment ; but it is only one out of 

 many among birds having the same breeding-range, not one 

 of which but shows variations of a parallel character, and 

 several of them in a nearly equal degree. It seems to me diffi- 

 cult to formulate the conditions whereby this tendency in so 

 many species — in all the species, we may say, of the region 

 under consideration — to vary in parallel lines and in the 

 same geographical direction, can be brought about by inter- 

 breeding, especially as these variations are strikingly correlated 

 with gradual changes in conditions of environment depending 

 upon differences of latitude and climate. The difficulty of 

 explaining all this on the theory of interbreeding becomes 

 still more e\ddent when we consider the adjoining regions to 

 the westward, where, as soon as we strike markedly different 

 conditions of environment, we meet with variations of a 

 somewhat different character, which again affect to a greater 

 or less degree all the species of the fauna, and are again cor- 

 related with a gradual modification of the environing condi- 

 tions. AYhen instances of intergradation between previously 

 supposed distinct species began to attract attention, hybridity 

 or interbreeding was the theory advanced for their explana- 

 tion ; but when it was found that variations of similar cha- 

 racter obtained among most of the species (of course in vary- 

 ing degree in different species) inhabiting the same areas, 

 and that certain phases of variation accompanied, or charac- 

 terized, or were correlated with, certain changes in the con- 

 ditions of life, the hypothesis of interbreeding was soon almost 

 wholly abandoned, and that of geographical or climatic varia- 

 tion adopted as being not only rational but obvious. 



While, from lack of requisite data, I cannot judge of the 

 particular cases cited by Mr. Seebohm in illustration of his 

 views, I cannot, on the other hand, subscribe to his belief 



