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ESSAY ON CLASSIFICATION. 



Part I. 



ization is higher in whicli the separate parts of an entire system differ more among 

 themselves, and each part has greater individuahty, than that in which the whole 

 is more uniform. I call type, the relations of organic elements and organs, as far 

 as their position is concerned. This relation of position is the expression of cer- 

 tain fundamental connections in the tendency of the individual relations of life ; 

 as, for instance, of the receiving and discharging poles of the body. The type 

 is altogether distinct from the degree of perfection, so that the same type may 

 include many degrees of perfection, and, vice versa, the same degree of perfec- 

 tion may be reached in several tyjjes. The degree of perfection, combined with 

 the type, first determines those great animal groups which have been called classes.^ 

 The confounding of the degree of perfection with the type of organization seems 

 the cause of much mistaken classification, and in the evident distinction between 

 these two relations we have sufficient proof that the different animal forms do 

 not jjresent one uniserial development, from the Monad up to Man." 

 The tyj^es he has recognized are : — 



I. The Peripheric Type. The essential contrasts in this type are between the 

 centre and the periphery.^ The organic functions of life are carried on in antag- 

 onistic relations from the centre to the circumference. Corresponding to this, the 

 whole organization radiates around a common centre. There exists besides only 

 the contrast between above and below, but in a weaker degree ; that between 

 right and left, or before and behind, is not at all noticeable, and the motion is 

 therefore undetermined in its direction. As the whole organization radiates from 

 one focus, so are the centres of all the organic systems arranged, ring-like, around 

 it, as, for instance, the stomach, the nerves and vessels, (if these parts are devel- 

 oped,) and the branches extending from them into the rays. What we find in 

 one ray is repeated in every other, the radiation being always from the centre 

 outwards, and every ray bearing the same relation to it. 



II. The Longitudinal Type, as observed in the Vibrio, the Filaria, the Gordius, 

 the Nais, and throughout the whole series of articulated animals. The contrast 

 between the receiving and the discharging organs, which are placed at the two 

 ends of the body, controls the whole organization. The mouth and the anus are 



' From this statement it is plain that Baer 

 has a very definite idea of the plan of structure, and 

 that he has readied it by a very ditierent road from 

 that of Cuvier. It is clear, also, that he understands 

 the distinction between a plan and its execution. 

 But his ideas respecting the different features of 

 structure are not quite so precise. He does not 

 distinguish, for instance, between the complication 



of structure as determining the relative rank of 

 the orders, and the different ways in which, and the 

 different means with which the plans are executed, 

 as characteristic of the classes. 



^ Without translating verbatim the descriptions 

 Baer gives of his types, which are greatly abridged 

 here, they are reproduced as nearly as possible in 

 his own words. 



