144 
modulating, and analysing, the sonorus vibrations before transmission 
to the appreciating centre, was pointed out as a condition that 
produced more remarkable gradations of development throughout 
the animal series, than the corresponding organ dealing with 
vibratile action, the eye, which directly conveyed the actual state 
of the rays of light, and was consequently more uniformly 
constructed. This difference gave rise to a particular interest in 
the survey of its structure. The gradations of form were not 
strictly in accordance with the degrees of development upon which 
the classification of animals were generally founded, but depended 
upon fluctuations in adaptation, and accommodation to the 
circumstances and period of life, specially in the earlier part of the 
series, that gave many variations and intermissions in the 
progressive advance. 
By the aid of many diagrams the earliest indications of the 
simple organ were demonstrated in Hydrozoa in medusal forms and 
acalepha, onwards through the groups Vermes, Mollusca, and 
Tunicata. It was shown that midst many variations there were 
fundamental principles of construction that were so persistent, that 
from their existence, the possession of an auditory organ and the 
sense of hearing could be predicated of the animals. This simple 
form consisted of a closed cavity containing solid grains, in contact 
with certain sentient bodies to which the filament of a special 
nerve were distributed. This early type, in certain limited variety, 
seem to obtain with animals leading a comparatively tranquil life 
in a confined area of space and medium. Considerable details were 
given in connection with these groups ; and the relation and develop- 
ment, collaterally associated, of the nerve system were entered into 
here as in subsequent groups. More marked changes were 
shown in the next group that was observed, Arthropoda. Many 
variations in the primary type were pointed out as associated with 
a changed character in the circumstances of life. The closed sac 
had an opening communicating with the external element, air or 
water, as the case might be ; and in insects, the chamber contained 
air and was fitted with a vibrating membrane stretched across an 
opening that communicated the vibration to the special apparatus 
within. 
The observations extended from the invertebrate to the verte- 
brate animals, in which the structures and organs, generally, 
advance in perfection and complexity, and present more varied and 
finished forms than heretofore, in conformity to more active and 
vigorous life under increased requirements and extension of space 
in movement. The simple form of the organ is changed, and there 
is no recurrence to it; constrictions extensions and additions take 
place, but the fundamental principles abide in the structure; the 
alterations mainly aiding the power of dealing, at first, with the 
