346 THE EVOLUTION OF THE METAZOA 



Evolution Generally Had a Progressive Trend 



Quite generally and, as it seems to me, correctly it has been 

 assumed that the evolution has generally taken a progressive 

 course. This, however, should not mean that there had been 

 no retrogressive evolution. Cases of distinctly retrogressive 

 phyletic evolution are well enough known and it is therefore 

 completely unnecessary for us to try to prove them in our 

 study. Whole classes have emerged as a result of .a retrogres- 

 sive evolution; yet at the same time we must be aw^are of the 

 fact that these retrogressions did not lead to reductions only, 

 but also to some new acquisitions which appeared as a con- 

 sequence of a specialized way of life. This was the same w^ay 

 of life which had caused a reduction of important organs 

 or systems of organs. An internal parasitism and a completely 

 sessile way of life have been the main cause of such a retrogres- 

 sive development (cf. Cestoda among the Ameria, Rhizo- 

 cephala among the Polymeria, and Tunicata among the 

 Chordoma). The question arises here whether any limit can 

 be found in such a development in the sense: what is the 

 highest systematic category where such a generally retrogres- 

 sive trend can still be observed? When we see that there are 

 not only classes (Dicyemidea, Cestoda) that had evolved by 

 way of a basically retrogressive development, but also sub- 

 phyla as this is proved by the case of the Tunicata, w^e cannot 

 exclude the possibility that even a w^hole phylum had eventual- 

 ly evolved by way of an evolution which shows a mainly 

 retrogressive trend. Later we will return to discuss this 

 problem more in extenso. 



Among other factors which must be kept in view when 

 we try to reconstruct the phylogeny of the animal world or 

 its genealogical tree that could serve as a basis of a natural 

 animal system, we must mention the various interpretations 

 which have been proposed regarding the origin ot large 

 groups. It is by no means surprising to see how^ wddely dif- 

 ferent opinions have been adopted by the zoologists on just 



