206 
ZOOLOGY. 
{Dendrceca), and otJier fly-ciitching warblers {MniotittidcB), 
many of whicli breed on the shores of Hudson's Bay, and 
spend the winter in Mexico or the West Indies. More spe- 
cies of birds breed in Canada than in the warm Southern 
States. Birds have been known to extend their range of 
migrations; the rice-bird or bobolink continually widens 
its range as rice and wheat are more extensively cultivated. 
This bird winters in Cuba and other West Indian islands, 
and probably also in Mexico. In April it enters the South- 
ern States and passes northward, till in June it reaches 
Canada and extends west to the Saskatchewan Eiver in 64° 
north latitude. 
Says Baird: While birds proceed generally in the spring 
to the very spot of birth, and by a definite route, their re- 
turn in autumn is not necessarily in the same line. Many 
birds are familiar visitors in abundance in certain locali- 
ties in either spring or autumn, and are not known there 
in the other season.'' He thinks that in very many in- 
stances birds proceed northward along the valley of the 
Mississippi, to return along the coast of the Atlantic. In 
general, also, the northward vernal movement is performed 
much more rapidly, and with fewer stops by the way, than 
the autumnal. Birds generally make their appearance in 
given localities with wonderful regularity in the spring — the 
SylvicoUdcB especially; a dilference of a few days in succes- 
sive years attracting the notice of the careful observer: this 
difference is generally influenced by the season. The time 
of autumnal return is, perhaps, less definite." (Baird.) 
While there are a number of very strange extinct birds, 
one of which called the Archmopteryx is the connecting 
link between reptiles and birds, and there are fossil birds 
with teeth, all the living species belong to two single sub- 
classes. 
Sub Classes of Existing Birds. 
1. Sternum smooth; wings rudimentary RatitcB (Ostrich). 
2. Sternum keeled: wings well developed. . . . Carinaice (Robin, etc.). 
