5 
specific members of a genus may be conceived as having descended from a 
common specific ancestor; the genera of a family from a common generic 
one, etc. 
Dealing only with existing North American birds, they may be divided 
into a number of Orders, which are the largest groups with which the 
Canadian ornithologist has direct concern. Orders are divided into 
Families , Families into Genera , and Genera into Species. These divisions 
may be again subdivided into Suborders, Subfamilies, Subgenera, and Sub- 
species whose positions in the scheme are evident from their titles. 
Though the limitations of book construction necessitate the presenta- 
tion of the classification scheme as a linear succession of forms following 
one another in single file, it should be borne in mind that the system is not 
linear in conception. The component species, instead of following a single 
line of relationship and sequence from the lowest to the highest, present 
many parallel or divergent lines of equal or subordinate rank. The class 
Aves or Birds may be represented by a tree, the height of the tree represent- 
ing time in geological ages from the earliest at the bottom to the present 
near the top. The trunk should be shown as double at the base; one sthm 
would be a short, dead stump and would represent the fossil, toothed 
birds which became extinct before present geological time; the other, 
large and thrifty, would represent the modern untoothed forms. This in 
turn would divide a short way from the base into two main branches 
to represent the two subclasses, the Raft-breasted and the Keel- 
breasted birds. The former would be represented by much the smaller 
branch, whereas the latter would divide and subdivide into branches repre- 
senting first, orders; next, families; then, genera; and finally, species. 
The value of these divisions — that is, the amount of differentiation 
sufficient to raise a group of genera to a family, or a collection of families 
to an order — is a matter for experienced individual decision, as there is no 
authoritative ruling upon the subject. However, there has gradually 
grown up an approximate agreement on this subject, though the constant 
tendency among specialists has been to make finer and finer distinctions 
and to multiply the number of the various groups. 
The smallest division generally accepted is the Species. Though 
everyone has a more or less accurate conception of what a species is, 
whether it be called by that name or another, no satisfactory definition 
has ever been constructed for it. It is what is commonly known as a 
“kind of an animal.” Thus the horse is a different “kind” or species from a 
donkey, a bluebird from a robin. They are sharply marked off from each 
other, regularly breeding only within the species and producing like 
species as offspring. Distinct species do not commonly interbreed; 
when they do so, they form crosses or hybrids that are usually sterile. 
Up to comparatively recent years no smaller division was recognized, but 
with intensive study of material it has become evident to advanced students 
that within the species there is considerable individual and geographical 
variation. 
Individual variation is the natural difference that may occur at any 
time between members of common parentage such as amongst full brothers 
and sisters. Just as like begets like, so within certain limits like begets 
unlike, for no two creatures are ever exact duplicates. This individual 
variation, usually small and irregular in appearance and direction, but 
in some cases persisting progressively generation after generation in one 
