110 



The progress of all sciences is a perpetual struggle after gene- 

 ralizations of a higher and higher order. Anatomy and physiology, 

 so actively cultivated in the time of Cuvier, had afforded at the 

 latter end of his career, glimpses of generalizations, which, under 

 the vague terms of " unity of organization," became subjects of 

 sharp controversy. The idea, so expressed, had two applications, — 

 one, to the analogies which exist between the permanent organiza- 

 tion of the lower animals, and certain transitory states of the higher 

 species ; the other, to the correspondences traceable between the 

 parts composing the organization of different species. 



With reference to the first of these applications, I cannot do 

 better than quote the author's own account of his conclusions, as 

 given in the last lecture of his course on the Invertebrate Animals, 

 published in 1843. 



" The extent to which the resemblance, expressed by the term 

 ' Unity of Organization,' may be traced between the higher and 

 lower organized 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 manifesta- 

 tion of the assimilative 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 organization 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 through 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 the unity of organization which prevails through the Polypes and 

 larval Acalephes, is diminished as the latter acquire maturity and 

 assume their special form. 



"There is only one animal form which is either permanently or 

 transitorily represented throughout the Animal Kingdom, — it is that 

 of the infusorial Monad. 



" Other forms are represented less exclusively in the development 

 of the Animal Kingdom, and may be regarded as secondary forms. 

 These are, the Polype, the Worm, the Tunicary, and the Lamprey ; 

 they are secondary in relation to the Animal Kingdom at large, but 

 are primary in respect of the primary divisions or sub-kingdoms. 



" Thus the Radiata, after having passed through the Monad stage, 

 enter that of the Polype : many there find their final development ; 

 others proceed to be metamorphosed into the Acalephe or the 

 Echinoderm. 



" All the Articulata, at an early stage of their development, assume 

 the form or condition of the apodal and acephalous worm ; some 

 find their mature development at that stage, as the Entozoa ; others 

 proceed to acquire annulations ; a head ; rudimental feet ; jointed 

 feet, and finally, wings : radiating in various directions and degrees 

 from the primary or fundamental form of their sub-kingdom. 



