CLASSIFICATION. _ 87 
to its investigation. Guided by 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 then- 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 groups, 
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 hi 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 living 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- 
