CONCLUDING SUMMARY. 645 



gations of the lower animals, and of the embryonic forms of the 

 higher ones, were forcing upon his reflective mind, if we may judge 

 from his struggles to express ideas, at that period so novel, and to 

 which he could but give imperfect utterance. " If we were capable," 

 he says, " of following the progress of increase of the number of the 

 parts of the most perfect animal, as they first formed in succession, 

 from the very first, to its state of full perfection, we should probably 

 be able to compare it with some one of the incomplete animals 

 themselves, of every order of animals in the creation, being at no 

 stage different from some of those inferior orders ; or, in other words, 

 if we were to take a series of animals from the more imperfect to the 

 perfect, we should probably find an imperfect animal corresponding 

 with some stage of the most perfect." * 



With the great accession of facts with which Comparative Anatomy 

 has since been enriched, particularly from monographs detailing dis- 

 sections of the Invertebrate animals, we may now attempt a more exact 

 enunciation of the resemblance which a higher organised animal pre- 

 sents to those of a lower order in its progress to maturity : and the 

 consequent extent to which the law of "Unity of Organisation " may 

 be justly, and without perversion of terms, be predicated of animal 

 structures. We shall see some grounds for the statement that the 

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

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

 correspondence does not extend to every oi^er of animals in the 

 creation.f 



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 approxima- 

 tion 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 acquires more conspicuously the 

 locomotive organs of the Monad, — the superficial vibratile cilia, — 

 before it takes on its special radiated type. The Acalephe passes 



* Hunterian MS. quoted in my ' Physiological Catalogue,' X, vol. i. p. 4. 



t This proposition is well supported and illustrated by Von Baer, CliXXXIX. 

 and CCLXX. 



t V. Baer, not recognising this phase of germ-life, affirms that " the Verte- 

 brata cannot descend to the lowest grades of organisation," and that their 

 embryos pass through no permanent forms of animals whatsoever. 



T T 3 



