128 CONCLUSION, 



and most wonderful of all animal structures. Without any other 

 reason, he is placed at the head of the great class of mammals ; 

 and it may be observed, as we advanced in our descriptions, that 

 the animals became less and less perfectly organised : the nervous 

 system, the respiratory and circulatory systems grow less and 

 less perfect as we proceed. On reaching the reptiles, we discover, 

 for example, that their heart has lost one ventricle, and their 

 blood is cold ; and in fishes, we find it has also lost an auricle, 

 and consists only of a single auricle, and a single ventricle. In 

 the fishes too, we observe striking changes in the composition of 

 bone ; and the skeleton gradually loses its hardness, until we 

 come to the Myxines, where it is soft and membranous ; or, it 

 might be said, the internal hard frame called the skeleton, has 

 disappeared, and we are brought step by step, to the considera- 

 tion of soft animals without vertebrae ; but, nevertheless, they 

 are provided with organs suitable for maintaining the kind of life 

 with which they are endowed by the Omniscient Creator of all 

 things. 



This concludes our account of the vertebrate animals, which 

 constitute the FIRST BRANCH of the Animal Kingdom, We 

 have next to consider the SECOND BRANCH, composed exclusively 

 of the Molluscous Animals, the study of which embraces Con- 

 chology, or the History of Shells. 



END OF THE FOURTH BOOK OF NATURAL HISTORY. 



