8 HOW BIRDS ARE NAMED 
our eastern bird, the former being no more a subspecies of the latter, 
than the latter is of the former. In other words, to continue to apply 
only generic and specific names to the Eastern Robin would imply that 
it was a species, while the use of a trinomial for the Western Robin 
would imply that it was a subspecies. Asa matter of fact, we know that 
there is but one species of true Robin in North America, consequently, 
in accordance with the logical and now generally accepted method, 
we apply to that species the name Planesticus migratorius, and this is 
equally applicable to Robins from the West and from the East. The 
eastern subspecies is, therefore, known by the trinomial Planesticus 
migratorius migratorius, the western subspecies by the exact nomencla- 
tural equivalent, Planesticus migratorius propinquus, and the more 
recently described small, pale Southern Robin, as Planesticus migra- 
torius achrusterus. Thus we have one Robin which is represented by 
three subspecies. It may be asked, why give names to these geograph- 
ical races? Why not call Eastern, Western and Southern Robins by 
one name without regard to their climatic variations? To which it 
may be replied, that subspecies often differ more from each other than 
do species. For example, it would clearly be inadvisable to apply the 
same name to the small, pale Song Sparrow of Arizona, and the large 
dark Song Sparrow of Alaska. (See figures beyond.) Seen without 
the connecting forms from the intervening regions and they apparently 
are specifically distinct, but the application to each of a subspecific 
name, or trinomial, not only indicates that they are different, but it 
shows also that they are representative forms which are joined by a 
series of intergrades; a contribution of the first importance to the study 
of evolution. 
For much the same reason, we should recognize by name those 
birds, which, like the Robin, show less pronounced climatic variations. 
Here we have species in the earliest stages of development from a 
common ancestor, and in naming them we are, in effect, giving ‘a handle 
to the fact’ of their evolution by environment. The study of the distri- 
bution and migration of birds and the mapping of natural life-areas are 
also intimately connected with this recognition by name of geographical 
variations. 
Since it is evident that a species may vary much or little according 
to the extent of its range, the governing conditions, and its tendency 
to respond to them, no fixed rule can be drawn which shall state just 
what degrees of difference are deserving a name. It follows, therefore, 
that in some cases ornithologists do not agree upon the validity of a 
bird’s claims to subspecific rank.* 
In North America, however, questions of this kind are referred to 
a Committee of the American Ornithologists’ Union, and its decisions 
establish a nomenclature which is accepted as the standard by other 
American ornithologists and which is adopted in this volume. 
Finally, the student should be warned not to permit this matter of 
*See Allen, Auk, 1890, pp. 1-9. 
