344 THE EVOLUTION OF THE METAZOA 



ontogeny of a certain animal species or of an animal group 

 had been changed so much that it no longer shows any trace 

 of recapitulations s, str. We believe that it is completely 

 improbable that cases could exist where up to 90 per cent 

 of characteristics that can be observed in an ontogeny could 

 be interpreted as true recapitulations. In this connection we 

 can mention the important difference which can be found in 

 the evaluations made by botanists regarding the frequency 

 of recapitulations that can be observed in plants. Broman 

 (1919) who studied this problem as an expert in genetics puts 

 the maximum at 30 per cent, and Zimmermann (1943) even 

 at 80 per cent. 



In future the symmetrical conditions should be studied more 

 critically than they have been till now, especially if we wish 

 to use them in our phylogenetic and systematic-taxonomic 

 constructions. Fortunately enough we can find that a con- 

 siderable progress has now been made in this direction. The 

 notion and the word Radiata and other similar names (Radialia, 

 etc.) should be avoided from now on. We soon find, if we 

 accept this suggestion, that the opposite symmetrical con- 

 dition, the bilateral symmetry and the corresponding category 

 Bilateria have also become unnecessary and that they should 

 also be abandoned. These symmetrical conditions are very 

 changeable. They depend entirely on the type of movement 

 or on the kind of contact w^hich exists between the animal 

 and its immediate environment. Not infrequently, we can find 

 different symmetries side -by-side in one and the same animal 

 group (taken phylogenetically they naturally occur one after 

 the other); some transitory forms have also become known. 



Bilateral symmetry must be considered a priori not only 

 as a characteristic but also as a primary property of animals 

 which are able to move, in opposition to plants which are 

 immobile. Radial symmetry, which in animals occurs only 

 rarely, is therefore a secondary phenomenon. It is certain 

 that a symmetry can be changed much more easily than the 

 whole structure. A transition from a horizontal (parallel to 



