TYPES OF CONSTRUCTION IN THE ANIMAL A' INC DOM. 233 



The motor organs, however, express their differentiation in 

 the skeletal parts and in the form of the body. There are 

 two types of differentiation of the motory system resulting in 

 the construction of what may be called muscular and skeletal 

 systems, or parts. Muscle and skeletal parts are correlative 

 to each other. Hard parts of some kind, to which the general 

 name skeletal is applied, arc essential to specialization of 

 the direction of motion, and contractile muscles are just as 

 essential to the motion of these skeletal parts themselves. 

 The relationship between these two elements of the motory 

 system is as intimate as that between steam and machinery 

 in the steam-engine. 



Archetypal Structure. — The further elaboration of this 

 method of analysis of organic structure may be pursued only 

 by tracing the elements of structure to their specific charac- 

 ters in many separate types. 



Sufficient may have been said to emphasize the fact that 

 there is a logical foundation for the idea of archetypal struc- 

 ture, so much insisted upon by Agassiz, explained embryo- 

 logically by Von Baer as early as 1828, and expressed in 

 Cuvier's classification of the Animal Kingdom, in the four 

 general plans upon which the various kinds of animals were 

 constructed. Cuvier wrote in i8i2- 



"... On trouvera qu'il existe quatre formes principales, 

 quatre plans gencraux, si Ton peut s'exprimer ainsi, d'apies. 

 ses quel tous les animaux semblent avoir ete modeles," etc. 



Cuvier's Classification. — Although, later, more minute stud- 

 ies have produced modification in the systematic classification 

 of the Animal Kingdom, Cuvier's division of animals into 

 Radiata, Articulata, Mollusca, Vertebrata, expresses the most 

 profound distinction exhibited by these organisms; and what- 

 ever criteria we take as the basis for classification, and with 

 slight modification due to increased knowledge, these grand 

 divisions of the Animal Kingdom stand out as pre-eminently 

 the most important groupings that can be made. 



To say so much is not an acceptance of the philosophy of 

 the earlier naturalists as final. That there are a few general 



*" Ann. des Mus d'Hist. Nalurelle," vol. xix., Paris, 1812, quoted by Agas> 



siz in ' An Essay on Classific:ilii/n," London, 1359, P- 3^9- 



