54 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



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

 arrangement of the viscera than we usually find in the higher Vertebrata. 

 This is evidently connected with their active locomotive powers ; as it is ob- 

 viously necessary that the two sides of the body should be balanced with per- 

 fect 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 ; 

 consequently 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 symmetrical than we usually 

 find it in the Mammalia. 



52. 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 Vertebrata ; 

 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 

 suspended in the midst of the white albumen, that, when the egg is laid upon 

 its side, it will always rise to the highest part of it; and the relative weight 

 of the several parts is further adjusted in such a manner, that the cicatricula 

 or germ-spot shall always be at the point nearest the shell, so as to come into 

 the closest proximity with the source of heat, and also to be in the most imme- 

 diate relation with the surrounding air. There are some birds, inhabiting the 

 equatorial regions, which do not always incubate their eggs, trusting to the 

 solar heat for their maturation. It is said that the Ostriches of the inter- 

 tropical deserts are content with covering their eggs with a thin layer of 

 sand, so as to admit the action of the sun by day, and to keep them warm at 

 night ; but that those living under a less constantly elevated temperature, sit 

 upon their eggs, if not constantly, at any rate when the solar heat is not suf- 

 ficient. This statement has been disputed; but its truth seems to be con- 

 firmed by a curious observation made by Mr. Knight, that a Fly-catcher, 

 which built for several years in one of his hot-houses, sat upon its eggs when 

 the temperature was below 72, but left them when it rose above that stand- 

 ard. The degree of assistance afforded by the parent Birds to their young, 

 after their emersion from the shell, varies much in different tribes ; in general 

 it may be remarked, however, that it is most prolonged in those which 

 ultimately attain the highest development, and especially in those whose intel- 

 ligence is the greatest. Thus the Chicken and the Duckling, when just 

 hatched, are able to shift for themselves ; but among the Raptorial and Inses- 

 sorial birds, which rank far higher in the scale, the young are for a long time 

 dependent upon the parent for food ; and in the Parrot tribe, which unques- 

 tionably surpasses all others in intelligence, the parent not only supplies its 

 young with food which it has obtained for them, but partly nourishes them by 

 a milky secretion from the interior of the craw ; impregnating with this the 

 aliment which it swallows, and which it afterwards disgorges for its offspring. 



General characters of Mammalia. 



53. The MAMMALIA are universally regarded as the highest group in the 

 Animal kingdom ; not only from being the one to which Man belongs (so far, 

 at least, as his bodily structure is concerned), but also as possessing the most 

 complex organization, adapted to perform the greatest number and variety of 

 actions, and to execute these with the greatest intelligence. The contrast is 

 here extremely strong between the reasoning and the instinctive powers ; even 

 when we put Man out of view. When we compare, for example, the saga- 

 city of a Dog, Monkey, or Elephant, and the great variety of circumstances 



