GENERAL CHARACTERS OF BIRDS. 



teeth, and in the development of a horny covering to the jaws) ; but in these, 

 the lateral elements of the sternum are the parts most developed, whilst in 

 Birds it is the central portion which exhibits the peculiarity. From the depth 

 of the keel of the sternum, a judgment may be formed of the thickness of the 

 pectoral muscles, and thence of the powers of flight ; in the Ostrich tribe, 

 where the wings are not sufficiently developed to raise the bird off the ground, 

 the sternum is quite flat, as in the Mammalia. The want of flexibility in the 

 trunk is counterbalanced by the length and flexibility of the neck ; the num- 

 ber of cervical vertebrae is very considerable, varying from 12 to 23, the 

 highest number being present in the Swan tribe. They are so articulated, 

 that the head can be turned completely round, or moved in any direction. 

 The anterior extremities of Birds being solely adapted to sustain them in 

 flight, the posterior are necessarily modified for their support on the ground. 

 They are usually placed rather far back ; but the spine has a position more 

 inclined than horizontal, so that the weight may not be altogether thrown for- 

 wards. The trunk is supported on the thighs by powerful muscles ; and 

 there is another series, which passes from the lower part of the spine continu- 

 ously to the toes, turning over the knee and heel, in such a manner that the 

 flexion of these joints shall tighten the tendons ; by this contrivance, the 

 simple weight of the body flexes the toes, and Birds are thus enabled to main- 

 tain their position during sleep, without any active muscular effort. 



50. Not only do Birds resemble Insects in their general structure and mode 

 of life, but also in the peculiar development of the instinctive powers. Under 

 the direction of these, the place for their nest appears to be selected; their 

 materials collected ; the nests themselves built, and the young reared in them ; 

 the migrations are performed; and many curious stratagems are employed to 

 obtain food. It is sufficient to indicate these in general terms; since it is well 

 known, that the habits of Birds have peculiarities restricted to each.species ; and 

 that in all the individuals of each species, they are as precisely alike as their 

 circumstances will admit. Nevertheless, there is observed in Birds a degree 

 and kind of adaptation to varying conditions, which Insects do not possess, 

 and which display an amount of intelligence far superior to what is found in 

 that class ( 280). This is evinced also in their educability ; for no animal 

 can be taught to perform actions which are not natural to it, unless it possesses 

 in a considerable degree the powers of memory and association, at least, if 

 not some of the higher mental faculties, such as the power of perceiving and 

 comparing the relations of ideas. Moreover, in the domesticability of many 

 tribes of Birds, \ve see this educability combined with a degree of that higher 

 form of attachment to Man, which is so strikingly exhibited by certain spe- 

 cies of Mammalia. The development of the senses of Birds varies in differ- 

 ent tribes, according to the mode in which they are adapted to obtain their 

 prey. The sight is almost always extremely acute, and is their chief means 

 of seeking food ; and where this would be of comparatively little service, as 

 in the nocturnal rapacious birds, it is compensated by a much higher deve- 

 lopment of the faculty of hearing than is usual amongst other tribes. The 

 senses of smell, taste, and touch, do not seem to be usually very acute in 

 Birds ; but there are particular tribes, in which each of these is more deve- 

 loped than in the rest. 



51. As might be expected from their analogy with Insects, the development 

 of the organs of nutrition (excepting that of the respiratory -organs) is much 

 less striking in Birds than is that of the locomotive apparatus. The whole 

 cavity of the trunk, especially in Birds distinguished for their powers of 

 flight, is small in comparison with that of the body ; but what is wanting in 

 the size of the organs, is made up in their energy of function. Hence the 

 demand for food is more active in them than in any other class of animals. 



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