10 INTEODUCTION. 



proper to the common ancestor of reptiles and birds alike. A 

 similar modification of the lung might thus be found in such 

 true reptiles as approach most nearly to birds ; and in fact we 

 see in the Chameleon (fig. 1) that long jthin air-sacs, connected 

 with the semi-spongy lungs, are suspended in the cavity of the 

 body, and may be directly compared with the large abdominal 

 air-saes which are found in all birds.' It is evident that, by 

 instituting such comparisons as these, we are tacitly ascribing a 

 character to the lungs of the Vertebrata which differs from 

 that we attributed to them when contrasting them with organs 

 of similar physiological function in land snails and land crusta- 

 ceans. For in the latter case we considered them, and with ' 

 justice, not as a character inherited from the parent form, and 

 as indicating near affinity, but as a character of adaptation, while 

 it is only among the Vertebrata that they are of real value in 

 estimating the degrees of affinity of the different classes. Thus 

 it is evident that in Vertebrata they possess all the significance 

 of hereditary characters, i.e. of parts which may be made use 

 of for investigating the evolution apd modification of these 

 classes — or, as may be, orders — one from another, and for 

 establishing such a natural system of the Vertebrata as may in- 

 dicate their true affinities. The same result is obtained when 

 the difierent organs of locomotion of the Vertebrata (wings, fins, 

 legs, feet, and hands) are taken into consideration. So long as 

 the comparison is extended to the whole cycle of the Verte- 

 brata, these seem to have the value merely of characters of 

 adaptation. The whale has fins as efficient as those of the stur- 

 geon or the pike, but I doubt whether a zoologist could be 

 found bold enough to attempt to derive the fins of the whale 

 morphologically from those of fishes. It is quite as unlikely 

 that anyone should undertake to prove that the wings of birds 

 or of bats (fig. 5) could have originated by direct modification 

 of the wing-like fins of flying fishes or of the dermal wings, 

 supported on ribs, of the flying reptiles (Draco). "With regard 

 to the higher classes of the Vertebrata all these organs are, 

 beyond a doubt, to be considered merely as characters of adapta- 

 tion, and so valueless for any determination of their affinity. 

 But if we now turn our attention to the same organs within 



