DEGRADATION OF ORGANISATION 75 



BIRDS. 



Animals without mammae, with two feet and two arms shaped as wings ; 

 the body covered with feathers. 



The second rank clearly belongs to the birds ; for while we do not 

 find among these animals so many faculties or so much intelhgenoe 

 as among the animals of the first rank, they are the only ones except 

 the monotremes which have hke mammals a heart with two ventricles 

 and two axiricles, warm blood, the cavity of the cranium completely 

 fiUed by the brain, and the trunk always enclosed by ribs. They have, 

 then, quahties common to mammals, but not found elsewhere ; and 

 consequently affinities with them that are not to be found in any 

 animals of the posterior classes. 



But the birds when compared with the mammals display an obvious 

 degradation of organisation which has nothing to do with the influ- 

 ence of the environment. They are for instance naturally devoid of 

 mammae, organs with which only animals of the highest rank are 

 provided and which belong to a system of reproduction that is no 

 longer found in the birds nor in any of the animals of subsequent 

 ranks. In short they are essentially oviparous ; for the system 

 of truly viviparous animals, which is adapted to animals of the first 

 lank, is not found in the second nor does it again re-appear. Their 

 foetus is enclosed in an inorganic envelope (the egg-shell) and soon 

 ceases communication with the mother and can develop without 

 feeding on her substance. 



The diaphragm, which among mammals completely separates some- 

 what obhquely the chest from the abdomen, here ceases to exist, or 

 becomes very incomplete. 



The vertebrae of the neck and tail are the only mobile parts in the 

 vertebral column of birds. Since movements of the other vertebrae 

 of that column are not necessary to the animal, they are not performed 

 and they thus place no obstacle to the large development of the 

 sternum which now makes such movement almost impossible. 



The sternum of birds indeed gives attachment to the pectoral 

 muscles, which have become very thick and strong by reason of their 

 energetic and almost continuous movements. The sternum has thus 

 become extremely large and carinate in the middle. This, however, 

 is due to the habits of these animals and not to the general degra- 

 dation that we are investigating. The truth of this is exemphfied by 

 the fact that the mammal called a bat has also a carinate sternum. 



All the blood of birds passes through their lungs before reaching 

 the other parts of the body. Thus they breathe exclusively by lungs 



