CNIDARIA AS THE ONLY COELENTERATA 45 



cases, too, the changes are connected with a change of the 

 environment. 



In the other extreme case, the main role in the life cycle is 

 taken over by the larval phase. The animal feeds during this 

 phase only. It lives in the larval environment for most of its 

 life. The imago stage is limited to a very short period of time; 

 it serves mainly the purpose of propagation and it's digestive 

 organs are frequently lost. Such cases are best known to occur 

 among insects. 



In our study of phylogeny and in our attempt to reconst- 

 ruct the phylogenetic development, we frequently meet with 

 the problem of w^hether a simple structure can be taken as a 

 sign of primitivism. This leads us to the important problem of 

 the progressivity of evolution. In a not too distant past it 

 could be frequently seen, and sometimes this occurs even 

 now, that a simple structure is taken simply as something 

 primary, primitive. Completely wrong constructions have been 

 made because of this mistake. It will be seen later that it was 

 especially so in the interpretation of the evolution of Cnidaria. 

 It must be confessed that it is not always very easy to diffe- 

 rentiate between a primary primitive and a secondary simp- 

 licity of a structure. Here one has to proceed very carefully; 

 one must make investigations by means of intensive and exten- 

 sive comparisons, taking particularly into consideration the way 

 of life and its changes. One must especially have a clear un- 

 derstanding of general conditions. Here it should suffice to 

 state that there certainly are both primary as well as secondary 

 simple phenomena, as has been proved by verified facts. This 

 is due to the fact that the evolution does not always and 

 exclusively show^ a progressive tendency ; its tendency can also be 

 in the opposite direction, it can be regressive, and this is quite 

 often the case. This regressive development (which is usually 

 and unfortunately called a degeneration) does not take place to a 

 minor extent only, i.e. in the lower categories of the animal 

 system. In my opinion, it has to be reckoned with even in 

 the so-called macro-evolution, or in the evolution of the 



