THE CLASSIFICATION OF THE VERTEBRATA, 183 



raising them to a group of equal value, as is now done under 

 the name of Ampliil)ia ; a word which I may mention was 

 used previously by Stannius to include the whole Reptilia of 

 Cuviert and Stannius was a good judge of the affinities 

 of Batracliia to other Reptilia, for the last edition of his 

 work exhibits enormously more knowledge of their anatomy 

 than has been shown in any book since published. 



Let us now look at some of the differences which make a 

 gap between the Fishes and, not the Amphibia only, but all the 

 animals above them. In the first place all fishes and only 

 the Fishes among Vertebrates possess a. heart consisting of one 

 series of chambers, a simple heart, receiving the blood into a 

 single auricle and propelling it thence into a ventricle, 

 which sends it into a single trunk, whence every drop has 

 to pass successively through two sets of capillaries. More- 

 over, although the swinnning bladder is undoubtedly 

 homologous with the lungs of other vertebrates, there is no 

 pulmonary artery carrying venous blood. But as soon as we 

 leave the Fishes we come to a construction common to all the 

 Cuvierian Reptiles in so far that Avhile the heart is more 

 complex, the work of circulation and respiration is not more 

 ofiectively accomplislied. A continuous structui-al evolution 

 is seen which beginning in the Amphibia rises in the croco- 

 diles to complete duphcity of the heart, similar to that of 

 the warm-blooded animals, but prevented from serving the 

 purposes of a completely double circulation by complications 

 in the arterial trunks. I repeat liere what I wrote in the 

 text-book already alluded to, that this complexity " though 

 in a manner accounted for as being a stage of progression 

 towards a more perfect organ found in higher animals, might 

 have been difficult to explain if it could have been noted by 

 an observer before birds and mammals appeared on the earth.' 

 This illustrates one of the characters of determinate evolutiou, 

 viz., that it cannot be appreciated till it is completed. 



A second great distinction between fishes and all other 

 vertebrates is to be found in the characters of their limbs. 

 There is great difficulty in making in detail the comparison 

 between the different parts of the limbs of fishes and 

 those of other vertebrates. The conclusions usually held 

 at the present time are not founded, I venture to assert, 

 on anything like a complete investigation of the subject ; 

 but this is not the place to enter on that question, and it 

 will serve my present purpose sufficiently to point out 

 that, when the limbs are developed in the non-piscine 







