76 ZOOLOGICAL PHILOSOPHY 



like the animals of the first rank ; and this is not the case with any 

 known animal after them. 



We now come to a very strange peculiarity which is connected 

 with the environment of these animals. They live more than other 

 vertebrates in the air, and are almost continually rising into it and pass- 

 ing through it in every direction. They have adopted a habit of swelling 

 their lungs with air in order to increase their volume and make them- 

 selves lighter ; and this habit has caused the organ to adhere to the 

 sides of the chest so that the air within, being rarefied by the heat 

 of the place, has had to pierce through the lung with its investing 

 membranes and to penetrate every part of the body even to the inside 

 of the great bones which are hollow, and to the quills of the large 

 feathers.^ It is, however, only in the lungs that the blood of birds 

 undergoes the necessary influence of the air ; for the air which 

 penetrates to the other parts of the body has another use than that 

 of respiration. 



Thus the birds, which have been rightly placed after the mammals, 

 exhibit an obvious degradation in their general organisation : not 

 because their lung has a peculiarity not found among the former, 

 for this is due like their feathers only to their acquired habit of launch- 

 ing themselves into the air ; but because they no longer have the 

 system of reproduction proper to the most perfect animals, but only 

 that which characterises most of the animals of the posterior classes. 



It is very difficult to ascertain among the birds themselves the 

 degradation of organisation which we are now studying ; our know- 

 ledge of their organisation is still too vague. Hence it has hitherto 

 been a matter of convention which order should be placed at the head 

 of this class and which at the end. 



We may reflect however that aquatic birds (like the palmipeds), 

 as also the waders and gallinaceans, have this advantage over all 

 other birds that their young on coming out of the egg can walk and 

 feed. We may pay special attention to the fact that among the pal- 

 mipeds, the penguins and king-penguins, whose almost featherless 

 wings are merely oars for swimming and of no use for flight, 



^ If it is true tliat in the case of birds the hings are pierced througli and the hair 

 changed into feathers as a result of their habit of rising into the air, 1 may be asked 

 why bats have not also feathers and pierced lungs. I re])ly that it seems to me pro- 

 bable that bats, which have a more perfect organisation than birds, and hence a 

 complete diajihragin to impede the swelling of tlicir lungs, have not been able to 

 pierce them through nor to swell themselves out with air sufficiently for that tluid 

 even by an effort to reach the skin and so to give to the horny matter of the hair 

 the faculty of branching out into featiiers. Among birds, in fact, air is introduced 

 as far as the hair l)ull)s ; changing their bases into quills and compelling this same 

 hair to break up into feathc^rs ; an event which -cannot occur in the bat, where tht* 

 air does not penetrate beyond the lung. 



