and Nature of Specific Distinctions. 507 



one general plan of structure, there cannot but appear some 

 tendency to converge to a general centre ; a tendency becoming 

 more obvious as we recede from the extremes, whereupon 

 there is usually a marked increase in the number of species 

 exhibiting the same characters, till at length a sort of focus 

 presents itself, as a central genus, the proper limits of which 

 completely baffle the ingenuity of naturalists to define, inas- 

 much as the various species it comprises blend with, and con- 

 tinuously radiate into, the immediately subordinate divisions. 



In illustration, it is sufficient to mention the already 

 cited genera, Corvus, ,4'rdea, Merula, and what should be 

 Pringilla, but which is at present better known as Cbcco- 

 thraustes. 



Take either of these divisions, and observe how difficult it 

 is to define its (artificial) boundaries ; how unbroken is the 

 concatenation of species which links them with what are simply 

 aberrant modifications of their structure, but which naturalists 

 have been accustomed to consider as separate and distinct 

 generic divisions. Let us, for a moment, consider ilierula. 

 Some naturalists try to separate the spotted-breasted thrushes 

 from those in which the markings are less broken ; and, un- 

 questionably, taking the extremes, there is much diversity ; 

 but there is quite as much between the different spotted- 

 breasted thrushes. In either case, however, where can the 

 dividing line be drawn? The blackbird has, when young, a 

 spotted breast; and, in fact, the characters of its nestling 

 plumage alone forbid its alienation from the spotted thrushes. 

 Where, indeed, can we trace the line of separation between 

 Merula and Philomela even ? And does not also the same 

 form, in another of its various gradations, merge imperceptibly 

 into Petrocincla, and thence into the different saxicoline 

 genera, jErythaca, Phcenicura, and Sialia? one ousel (Petro- 

 cincla, or, rather, Geocincla Gould) being absolutely a large 

 robin, another a great redstart, while a newly discovered 

 species of Sialia has the markings, and many of the characters, 

 of a Petrocincla? But it would be endless to follow iVferula 

 into all its diversified ramifications. I shall content myself 

 with tracing the series into Philomela, which is at once con- 

 clusive as to the true affinities of the latter. 



To be brief, then, we observe in the European song 

 thrush a deviation from the gregarious character of its nearest 

 British congeners, and an approximation to the style of 

 marking in the transatlantic species. M. ?mistelma of North 

 America is yet more solitary, and does not even associate to 

 migrate ; in this resembling Philomela, which its habits (as 

 described by all who have observed them) accord with in 



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