20 INTRODUCTION. 



likewise the Turdidce consist of the Turdince, or typical 

 Thrushes, who have a " booted " tarsus, and the Mimince*^ or 

 Mocking-thrushes, who do not possess this feature. Combinan 

 tions of less important distinctions in structural detail char- 

 acterize the genera; and therefore birds belonging to the 

 same genus have exactly the same structure. The difference 

 between species is marked by coloration, and often size. To 

 illustrate the foregoing, take as a subject the common Song 

 Sparrow. He belongs to the Insessorian group, the order of 

 Passeres, and the suborder Oscines. His family is that of 

 the Fringillidce, or Finches, and his genus Melospiza. His 

 specific name isfasciata, but there is also a variety, M.fallax. 

 " Extremely similar ; wings and tails slightly longer ; paler, 

 grayer ; the streaks not so obviously blackish in the centre. 

 Whole of the Rocky Mountains and Great Basin ; scarcely 

 distinguishable." (Coues.) 



The classification of birds (or other objects in nature) 

 necessarily entails certain absurdities, being more or less artifi- 

 cial. The method of modern classification is that of descend- 

 ing from the higher to the lower groups, but sequence is too 

 often insisted on among groups which diverge (so to speak) 

 from a common centre. The division of families, genera, etc., 

 is more or less arbitrary, a certain degree of difference being 

 necessary to separate them, whereas minor (or fractional) 

 differences cause intermediate groups. Let x represent the 

 degree of dissimilarity between two genera ; then — repre- 

 sents the difference between two subgenera (of the same ge- 

 nus) ; 1 + ^ between subfamilies, 2x between families, 2 +- 

 between suborders, and 3 x between orders. Forms in nature 

 are everywhere so delicately blended that theoretically it is 

 almost impossible to "draw any lines." A species (even 

 though exhibiting much individual variation) is constituted 

 by all those birds who, since the present organization of birds 

 has existed, might be the descendants of a single pair. 



[The phenomena of albinism and melanism can here be only 

 briefly referred to. In the latter, which is very rare, birds are 



* American omithologists now consider the Mimitm a subfamily of tlie 

 Troglodytid<B or Wrens. — W. B. 



