86 INTRODUCTION. 
red almost obsolete, the wings sometimes spotted, and the 
black of the head pale. Perhaps it is a much smaller 
bird than the average, but no one thinks of calling this 
a “new species”; although, if it were not for the fact 
that there are Robins presenting every shade in color and 
difference in size between this and the typical specimen, 
it would certainly be a good species. Why are not the 
same variations, which we can here see at a glance, dis- 
covered in the other members of this family? They cer- 
tainly exist. But more of this anon. 
The Robin has also characters that it bears in common 
with other Z'urdine, which are its true generic characters. 
If, then, we cannot establish a connecting link in the man- 
ner described between one species and its nearest allies, 
we may be sure that it never * mixes with others in breed- 
ing, but always mates with one having the same pecu- 
liarities as itself, although changeable to a certain point. 
This constitutes a natural species. If, on the other hand, 
we do find a connecting link, many times repeated in differ- 
ent individuals, between a supposed species and its nearest 
ally, we may be sure that they are one. 
In the succeeding pages I have followed the classifica- 
tion of Professor W. Lilljeborg, of Upsala, as adopted pro- 
visionally by the Smithsonian Institution. The original 
method being the ascending or progressive mode, while 
the one used is the descending mode, with other minor 
changes. 
This classification is by far the best in use, although, 
perhaps, not perfect. By the old methods the Vulture, 
vile feeder of carrion, was placed first, and we were told 
to look to him as king of the birds.) But King Vulture 
has been dethroned, and in his stead reigns the Thrush 
king over all; crowned for his sprightly intelligence and 
* The well-known exceptions to this rule are Colaptes auratus mixing 
with C. Mexicanus; two of the Juncos, and perhaps others 
