368 LECTUltE XXIV. 



more perfect animal is at no stage of its development different from 

 some of the inferior species ; but we shall obtain proof that such cor- 

 respondence does not extend to every order of animals in the creation. 



The extent to which the resemblance, expressed by the term ' Unity 

 of Organisation,' may be traced between the higher and lower organised 

 animals, bears an inverse ratio to their approximation to maturity. 



All animals resemble each other at the earliest period of their 

 development, which commences with the manifestation of the as- 

 similative and fissiparous properties of the polygastric animalcule : 

 the potential germ of the Mammal can be compared, in form and 

 vital actions with the Monad alone, and, at this period, unity of 

 organisation may be predicated of the two extremes of the Animal 

 Kingdom. The germ of the Polype pushes the resemblance farther, 

 and acquires the locomotive organs of the Monad, — the superficial 

 vibratile cilia, — before it takes on its special radiated type. The 

 Acalephe passes througli both the Infusorial and Polype stages, and 

 propagates by gemmation, as well as spontaneous fission, before it 

 acquires its mature form and sexual organs. The fulness of tiie 

 unity of organisation which prevails through the Polypes and larval 

 Acalephes, is diminished as tiie latter acquire maturity and assume 

 their special form. 



The Ascidian MoUusks typify more feebly and transiently the polype 

 state in passing from that of the cercarii-form ciliated larva to the 

 special molluscous form. The Gasteropods and Bivalves obey the law 

 of unity of organisation in the spontaneous fissions of their amorphous 

 germ, and in its ciliated e ithelium, by which it gyrates in the ovum ; 

 but they proceed at once to assume the molluscous type without 

 assuming that of the Polype ; the Bivalve retaining the acephalous 

 condition, the Univalve ascending in its development to the acqui- 

 sition of its appropriate head, jaws, and organs of sense. 



Thus all moUusks are at one period like Monads, at another Acepha- 

 lans ; but scarcely any typify the Polypes, and none the Acalephes. In 

 the Encephalous division we meet with many interesting examples of 

 the prevalence of unity of organisation at early periods, whicli is lost in 

 the diversity of the special forms as development proceeds. Thus the 

 embryos of the various orders of Gasteropods are nudibranchiate ; but 

 only a few retain that condition of the respiratory system through 

 life. The naked Gasteropods are at first univalve MoUusks, like the 

 great bulk of the class at all periods. The testaceous Cephalopods 

 first construct an unilocular shell, which is the common persistent 

 form in Gasteropods, and afterwards superadd the characteristic 

 chambers and siphon. This simple fact would of itself have disproved 

 the theory of evolution, if other observations of the phenomena of 



