2 INTRODUOTOET. 



birds, the lungs are spongy, the cavity of the air-bags becoming 

 obliterated hj the multiplication of vascular cellules ; the heart 

 is four-chambered, transmitting venous blood to the lungs, and 

 pure arterial blood to the body; the temperature is raised and 

 maintained at 90'=' to 100^ Fahr. 



Thus Reptiles, like Birds, breathe the common air by means 

 of their lungs, but respiration is much less active. " Although," 

 remarks Professor Owen, " the heart of Birds resembles in some 

 particulars that of Eeptiles, the four cavities are as distinct as 

 in the Mammalia, but they are relatively stronger, their valvular 

 mechanism is more perfect, and the contractions of this organ 

 are more forcible and frequent in birds, in accordance with 

 their more extended respiration and their more energetic mus- 

 cular action." It is true, as Profcssoi' Huxley informs us, that the 

 pinion of a bird, "which corresponds with the human hand or the 

 fore paw of a reptile, has three points representing three fingers : 

 no reptile has so few.* The breast-bone of a bird is converted 

 into membrane-bone : no such conversion takes place in reptiles. 

 The sacrum is formed by a number of caudal and dorsal vertebras. 

 In reptiles the organ is constituted by one or two sacral vertebrce. 



In other respects the two classes present many obvious dif- 

 ferences, but these are more superficial than would be suspected at 

 first glance. And Professor Huxlej' believes that, structurally, 

 "reptiles and birds do reallj^ agree much more closely than birds 

 with mammals, or reptiles Avith amphibians." 



While most existing birds diifer thus widelj- from existing rep- 

 tiles, the cursorial or struthious genera, comprising the Ostrich, 

 Nandou, Emu, Cassowary, Apteryx, and the recentlj- extinct 

 Dinornis of New Zealand, come nearer to the reptiles in structure 

 than any others. All of these birds are remarkable for the short- 

 ness of their wings, the absence of a crest or keel upon the breast- 

 bone, and some peculiarities of the skull, which render them more 

 peculiarly reptilian. But the gap bet-\vecn reptiles and birds is 

 only slightly narrowed by their existence, and is somewhat unsatis- 

 factorjr to those who advocate the development theory, which 

 asserts that all animals have proceeded, by gradual modification 

 from a common stock. 



* i'icfc, ho\Yevcr, p. 8. — Eu. 



