CLASSIFICATION. g^ 



to its investigation. Guided hj only a very limited 

 knowledge of their structure, and observing, -what is 

 very obvious, that there is a superior and an infe- 

 rior extreme in their organization, and a gradation 

 from the one point to the other, it was inferred that 

 they are all arranged in a regularly descending series, 

 and constitute an unbroken chain from the highest to 

 the lowest. It was seen after a time, however, that 

 this is true only in a very general sense, and that it 

 regards the great classes rather than inferior gi'oups, 

 the degrees of the scale being very irregular, and the 

 intervals between some of them of vast extent. Such 

 a hiatus occurs between the two great divisions we have 

 just named, the distance between the animals lowest in 

 rank in the vertebral classes, and the most highly or- 

 ganized invertebral animals being almost infinite. In 

 passing from one to the other the transition is immediate 

 from beings possessing a brain and highly developed nerv- 

 ous system, distinct organs of sense, an internal bony 

 skeleton which is a part of the hving structure, renewed, 

 from time to time, like the other tissues of the body, 

 and extensive powers of locomotion, and endowed, more- 

 over, with intelligence, to those in which there is neither 

 a brain nor cerebral nerves, where the organs of sense 

 are for the most part wanting or singly or doubtfully 

 developed, the body composed of a soft and flaccid sub- 

 stance protected in some classes by an external cover- 

 ing of hard and dense structure, but distinguished from 

 bone by its want of vitality, and the power of locomo- 



