Page 252 - Third Book of Reading Lessons

READING LESSONS. 251
the base of the beak: the feet are armed with strong nails. They Iive on esh, and pursue other birds ; hence, they have generally a power l ight. The greater number have, moreover, a small web between the external toes.
6. The passerine bfrds asser, a sparrow) include many more species than all the other milies ; but •the analogy in their organisation is so great, that they cannot be separated, although they va greatly in size and strength. Their two external toes are united at the base, and sometimes part of the way up their length. Each of these orders subdivides into milies and genera, or kinds, principally by the
con rmation o the beak.
7. Birds are, in general, covered with feathers, a sort of tegument the best adapted to protect them from the e ects of the rapid variations of temperature to which their movements expose them. The air­ cavities which occupy the interior of their body, and w ich even occupy the place of marrow in the bones, augment their speci c lightness.
8. Sight is extremely per ct in birds, and they have the peculiar culty of seeing objects near or distant equally well. The means by which this is e ected are not satis ctorily explained, though a power of changing the convexity of the eye is prob­ ably the proximate or immediate cause. Like all other physical peculiarities, it is admirably adapted to the mode of existence of the class: a quick and per ct sight of objects and perception of distances are necessary to the rapidity of their movements and
the securing of their prey to birds. All the gen except the owls, see a single object but with one y The situation of these organs, however, enable&±liw