124 ON THE INSTINCTS OF BIRDS. 
tion for several years with comparatively little inter- 
ruption. A check to the greatly increased activity of 
the reproductive powers so induced is speedily given, 
however, by a diminution of sustenance and exposure 
to cold; at the same time, also, a visible change 
takes place in the physical condition of the organs of 
reproduction. 
In the selection of their mates, the feathered tribes 
are undoubtedly governed by instinct, as there is reason 
to believe that different species, in a state of nature, 
rarely pair together, however near their affinity or 
general resemblance may be. The Rook is not ob- 
served to breed with the Crow, the Titlark with the 
Lesser Field-Lark or Rock-Lark, the Sedge-W arbler 
with the Reed-Wren, or the Cole Titmouse with the 
Marsh-Titmouse. Now, were every individual left to 
the unrestrained exercise of its own discretion in a 
matter of such essential importance, the utmost con- 
fusion might be expected to ensue; an unprolific, 
hybrid progeny would be speedily produced, and the 
total extinction of many species might be the ultimate 
consequence. But the allwise Author of nature has 
not suffered the reproduction of his creatures to be 
liable to such a contingency, but has implanted in 
the mind of each a powerful predisposition to form 
sexual unions with its own kind exclusively. Thus 
the evils which would unavoidably result from the 
indiscriminate intercourse of various species are effec- 
tually prevented. 
