440 THK EVOLUTION OF THE METAXOA 



which can be solved more easily— of how to explain the 

 evolution of the polymerous state of the Chordonia from an 

 oligomerous state with three segments at the most and with 

 the same number of pairs of the enterocoele (the anterior 

 pair can also be found grown together). 



We must include here first a general observation before 

 we try to solve this difficult problem. The number of the 

 main types of life and of the corresponding main forms of 

 life is very limited. We can therefore see them repeated again 

 and again within the framework of all the higher groups. 

 The products of evolution, however, show as parallelisms 

 considerable differences in spite of frequent analogies and 

 external similarities; this is due to the fact that they always 

 start their evolution from different initial points (levels of 

 organization). Because of these similarities and analogies it 

 is frequently difficult to identify the initial point of evolution. 

 In our attempt to solve the problem of the origin of the 

 Chordata we meet with one of the most important repetitions, 

 the repetition of the body segmentation. It first took place 

 by way of an irregular polymerization finally, to reach a regular 

 polymery during the transition from the Ameria to the Poly- 

 meria; it was a consequence of a rapid type of movement by 

 means of a curving of the entire body. Polymery was sub- 

 sequently reduced to an oligomery within the framework of 

 the Oligomeria. During the transition from the Oligomeria 

 to the Chordonia we can again observe an irregular poly- 

 merization which led to polymery. And again this had been 

 caused by an active type of movement. In this case, however, 

 we can hardly speak of a sinuous type of movement; here 

 the waves of contractions are very low, and the segments 

 of muscles involved in this movement very short, following 

 each other closely; they are limited to the dorsal side. The 

 effect of these contractions had therefore been greatly increas- 

 ed. Later we can again observe in the Chordonia a secondary 

 reduction of the number of segments, above all among the 

 Tetrapoda where the reduction was caused especially by the 



