GENERAL DISTRIBUTION. 
175 
certain species is co-extensive with one or the other of 
the earth’s zones, the great similarity of birds inhabiting 
the districts of two corresponding zones being unmis¬ 
takable. The Great Auk (Alea impennis ) of the North 
Pole, and the Penguin, inhabiting the islands of the 
south Polar seas, are proofs of this; differing, as they do, 
one from the other, they cannot be looked upon as other 
than closely allied. 
On the other hand, the sea gives no examples of 
limited distribution, except only on small islands, where 
a singular instance, of a very limited field of distribution, 
is to be met with : in the Galapagos group situated under 
the equator, 500 nautical miles west of America, Darwin 
discovered six-and-twenty different species of birds, of 
which only one had ever been observed elsewhere. 
In a similar manner, also, Australia and Madagascar 
produce species peculiar to themselves. Many islands, 
situated on the glassy bosom of the ocean, possess birds 
which are not to be met with upon other islands only a 
few miles off. Such decided lines of demarcation are 
not found on the main land. The more perfectly- 
developed sea-birds form quite a contrast to the isolated 
birds of these islands. Among the mighty flyers, namely, 
the Giant Petrel (.Procellaria gigantea ) and Albatross have 
no distinct circle of distribution, though originally 
emanating from—and breeding in—the torrid zone. 
They wander over all the seas of the earth, and cover 
such enormous distances in a day’s flight that no distinct 
limits can be assigned to their habitats. 
The total number of species of birds, hitherto dis¬ 
covered, amounts to nearly 8000:* of these, the Parrot 
* This estimate is certainly under the mark. The latest computation makes the 
number of species known 11,162 (vide Gray’s ‘Hand-list of Birds’); and, allowing 
for every contingency, the number of known birds must be about 10,000.— W. J. 
