506 Royal Institution. 



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 show always means, merely, 

 ** a single thing of a given kind." 



There are, however, several kinds of Individuality. 



Firsty 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. Phsenomena 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- 

 ganized being has been formless and will again be formless ; the indi- 

 vidual animal or plant is the sum of the incessant changes which 

 succeed one another between these two periods of rest. 



The individual animal is one beat of the pendulum of life, birth 

 and death are the two points of rest, and the vital force is like the 

 velocity of the pendulum, a constantly varying quantity between 

 these two zero points. The difi'erent forms which an animal may 

 assume correspond with the successive places of the pendulum. 



