GENERAL CHARACTERS OF BIRDS. 61 



continuously 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 

 maintain their position, by grasping their perch, during sleep, without any 

 active muscular effort. 



39. 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 nests appears to be selected ; 

 their materials collected ; the nest themselves built, and the young reared in 

 them ; the migrations are performed ; and many curious stratagems are em- 

 ployed 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 are ob- 

 served in Birds a degree and kind of adaptation to varying conditions, which 

 Insects do not possess, and which display an amount of intelligence far su- 

 perior to what is found in that class ( 17). This is evinced also in their edu- 

 cubility ; 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, we see this educability com- 

 bined with a degree of that higher form of attachment to Man, which is so 

 strikingly exhibited by certain species of Mammalia. The development of 

 the senses of Birds varies in different 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 compen- 

 sated by a much higher development of the faculty of hearing, than is com- 

 mon 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 developed than in the rest. 



40. As might be expected from the analogy of Birds with Insects, the de- 

 velopment of their organs of nutrition (excepting that of the respiratory organs) 

 is much less striking J,han 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. 

 It is interesting to observe, that there is more bi-lateral symmetry in the 

 arrangement of the viscera, than we usually find in the class above. This is 

 evidently connected with their active locomotive powers ; as it is obviously 

 necessary, that the two sides of the body should be balanced with perfect 

 equality, and that their energy should be exactly correspondent. The lungs 

 and air-sacs are precisely similar in size and situation on the two sides ; con- 

 sequently the heart is placed on the median line ; and the mode of origin, 

 from the aorta, of the trunks supplying the head and upper extremities, is 

 alike on the two sides. The liver, also, is less asymmetrical than we usually 

 find it in the Mammalia. 



41. It has been remarked, that the assistance afforded by the parent, in 

 the development of the young, is greater in Birds than in the lower Verte- 

 brata ; but is less than in Mammalia* Whilst Reptiles and Fishes show little 

 or no concern for their eggs after they have deposited them, Birds sedulously 

 tend them, affording them not only protection but warmth, by means of their 

 powerful heat-producing apparatus. The yolk-bag of the Bird's egg is so 



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