PRINCIPAL SYSTEMS OF ZOOLOGY 247 



ness of the elementary parts, and the separate divisions of a compHcatd appa- 

 ratus, — in one word, in the greater histological and morphological differentia- 

 tion. The more uniform the whole mass of the body is, the lower the degree of 

 perfection; it is a stage higher when nerve and muscle, blood and cellular tissue, 

 are sharply distinguished. In proportion to the difference between these parts, 

 is the development of the animal life in its different tendencies; or, to express it 

 more accurately, the more the animal life is developed in its several tendencies, 

 the more heterogeneous are the elementary parts which this life brings into action. 

 The same is true of the single parts of any apparatus. That organization is 

 higher in which the separate parts of an entire system differ more among them- 

 selves, and each part has greater individuality, 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 per- 

 fection may be reached in several types. The degree of perfection, combined with 

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

 classes. 54 The confounding of the degree of perfection with the type of organ- 

 ization seems the cause of much mistaken classification, and in the evident dis- 

 tinction between these two relations we have sufficient proof that the different 

 animal forms do not present one uniserial development, from the Monad up to 

 Man. 



The types 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 antagonistic relations from the centre to the 

 circumference. Corresponding to this, the whole organization radi- 

 ates around a common centre. There exists besides only the contrast 

 between above and below, but in a weaker deoree; that between risht 

 and left, or before and behind, is not at all noticeable, and the mo- 

 tion is therefore undetermined in its direction. As the whole organ- 

 ization 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 developed) and the branches 



"From this statement it is plain that von Baer has a very definite idea of the plan 

 of structure and that he has reached it by a very different 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 determin- 

 ing 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 von Baer gives of his types, which 

 are greatly abridged here, they are reproduced as nearly as possible in his own words. 



