OF NORTHERN ILLINOIS. 51;5 



specimens from distant localities have led to the formation of new species on unwarrantable 

 grounds. From this it is evident that an ornithologist should thoroughly acquaint him- 

 self with the amount of the variation in the proportion of parts, as well as in color, and with 

 the laws governing the successive changes in color presented by the plumage during the 

 whole life of the bird, as also the seasonal changes which the birds about him present, 

 before he is fully qualified to deal with those from distant regions, of which he can see but a 

 limited number of prepared specimens. Besides the variations presented by the representa- 

 tives of any species at the same locality, there are others that may be termed geographical, 

 depending as they do upon variation in the nature of the habitat, the exact character and 

 extent of which we as yet know little; but that nearly every species occupies in the breed- 

 ing season a wide range in latitude, and in many cases markedly so, has already been too 

 fully demonstrated — thanks especially to the thorough elaboration by Prof. Baird and his 

 collaborators, of the immense collections of North American birds brought together at the 

 Smithsonian Institution — to admit of reasonable doubt. 1 As to what other characters also 

 vary with the locality, we have less positive assurance ; though the paler color of specimens 

 coining from the Middle or Rocky Mountain Plateau region of the continent, in species 

 ranging from the Atlantic to the Pacific, compared with specimens from the Atlantic or 

 Pacific slopes, is now known in too many species 2 to be considered as an accidental circum- 

 stance. These facts of variation should not, it is evident, lose their weight in considerations of 

 specific diversity in obscure cases, and especially when the specimens are from new or little 

 known localities. Yet the geographical difference in size is even now occasionally, but of 

 course blamably, overlooked by not a few naturalists in their eagerness to add a few more 

 " new species " to their lists. 



Species when properly limited, or species in nature, we believe to be tangible things ; but 

 many of the zoologists of to-day are making them the most inconstant and intangible of all 

 objects — a mere matter of opinion and locality, in fact, — of which the Thrushes under con- 

 sideration, and especially the American Turdidce generally, furnish a good, but, unfortunately, 

 not a solitary illustration in American birds. If we admit seven species in the group in ques- 

 tion as inhabitants of the United States, I see no reason why. on similar grounds, we may not 

 further increase the number. But if we restrict them to three — Turdus Pallasi, T. Swainsoni, 

 T. fuscescens, — we have three that are well marked and well separated, with rarely, if ever, 

 a specimen occurring that may not with the greatest confidence be referred to one or the 

 other. 



We have already seen (antea, pp. 508-9), that individuals of Turdus Swuinsoni occur cor- 

 responding in size with the typical or original specimens of the so-called species '• Turdus 

 nanus" and Turdus ustulutus, affording parallel grounds for an additional species, which shall 

 hold the same relation to T. Swainsoni that these do respectively to T. Pallasi and T. 

 fuscescens. Once overstepping, then, the restrictions given here, we may go still further, and 

 taking the extremes of size and of color, and the means, have not seven only, nor merely 

 nine, but eighteen species ! since the extremes and the means in size have each their dark 

 and light colored representatives, depending upon the intensity of the rufous tint. Palpably 



1 For the first formal statement of this law of variation in a Among these species we may enumerate Poceceles gratn- 



size with latitude, see Prof. Baird on the " Distribution and ineus, Sturnella m ign i, Chordeiles pnpetue, Eremophila 



Migration of North American Birds." A?n. Joum. of Sc. and alpesiris, Bubo virginianus, liutco boreal is ; and probably add 



Arts, [2] vol. xli., March 186G. Turdus Pallasi ("Auduboni"), and perhaps the other 'I'urdi. 



MEMdins BOST. SOC. NAT. HIST. Vol. I, Pt. 4. 130 



