1852.] OF THE ROYAL INSTITUTION. 185 
Each of these carries a single ovum, from which the first form is 
again developed. 
The Salpa mucronata, which is thus produced from the Salpa de- 
mocratica, is just as highly organized as the latter. It has as com- 
plete a circulatory, nervous, and digestive apparatus, and moves 
about and feeds as actively ; no one unacquainted with its history 
would dream of its being other than a distinct individual animal, and 
for such it has hitherto passed. 
But the Salpa mucronata has exactly the same relation to the S. 
democratica that the free-medusiform egg-producing body of Physalia 
or Velella has to the Physalia or Velella; and this free-medusiform 
body is homologous with the fixed medusiform body of Diphyes ; 
which again is homologous with the semi-medusiform, fixed body of 
a Tubularia and with the egg-producing process of the Hydra. 
Now as all these bodies are homologous with one another, one of 
two conclusions is possible; either, considering the Salpa mucronata 
to be an individual, we are logically led to look upon the egg- 
producing process of Hydra as an individual also, which seems 
absurd. 
Or starting with the assumption that the egg-producing process 
of Hydra is a mere organ, we arrive at the conclusion that the Salpa 
mucronata is a mere organ also: which appears equally startling. 
The whole question appears to. turn upon the meaning of the word 
“ individual.” 
This word the Lecturer endeavoured to shew always means, merely, 
“a single thing of a given kind.”’ 
There are, however, several kinds of Individuality. 
Firstly, there is what may be called arbitrary individuality, which 
depends wholly upon our way of regarding a thing, and is therefore, 
merely temporary: such is the individuality of a landscape, or of a 
period of time ; a century for instance. 
Secondly, there is an individuality which depends upon something 
else than our will or caprice; this something is a fact or law of co- 
existence which cannot be materially altered without destroying the 
individuality in question. 
Thus a Crystal is an individual thing in virtue of its form, hard- 
ness, transparency, and other co-existent qualities ; pound it into 
powder, destroy the co-existence of these qualities, and it loses its 
individuality. 
Thirdly, there is a kind of individuality which is constituted and 
defined by a fact or law of succession. Phenomena which occur 
in a definite cycle are considered as one in consequence of the law 
which connects them. 
As a simple instance we may take the individuality of the beat of 
a pendulum. An individual beat is the sum of the successive places 
of the bob of the pendulum as it passes, from a state of rest to a 
state of rest again. 
Such is the individuality of living, organized beings. Every or- 
