20 INTRODUCTION. 



likewise the Turdidce consist of the Turdince^ or typical 

 Thriislies, who have a " booted " tarsus, and the Miminoe^* or 

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

 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 sjyecies is marked by coloration, and often size. To 

 illustrate the foregoing, take as a subject the common Song 

 Sparrow. He belongs to the Insessof'ian group, the order of 

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

 the Fringillidoe^ or Finches, and his genus Jlelosjyiza. His 

 specific name is fasciata, 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-f-— between subfamilies, 2x between families, 2 -|-^ 

 between suborders, and 3 a? 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, atid melanism can here be only 

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



* American omitholog-ists now consider the Miminm a subfamily of the 

 Troglodytidce. or Wrens. — W. B. 



