280 THE EVOLUTION OF THE METAZOA 



It cannot be emphasized strongly enough that quite naturally 

 the recent Infusoria with their frequently extreme specializa- 

 tions cannot be taken into consideration as ancestors of the 

 equally recent Acoela. The time when the line of Turbellaria 

 had supposedly branched off from its infusorian stem lies far 

 back in geologic time. It is even questionable whether a name 

 like infusorian, or the acoelous turbellarian, could still be given 

 to this common original form, by a modern zoologist. It is 

 quite irrational and beyond understanding to expect or to 

 require that some recent transitional forms should be shown 

 to those who oppose our thesis, as proofs of this development 

 which has been so important for the evolution of the animal 

 world. The phylogeneticists have repeatedly emphasized that 

 such fatal steps in evolution have frequently taken place much 

 faster than did the development of some less important 

 "novelties" and the evolution of large types. 



The Relationship Between the Conjugation 

 and the Copulation 



First a problem of terminology. The word "copulation" has 

 unfortunately been used for two entirely different phenomena. 

 On one hand, it is a union of the gametes of the two sexes, or 

 the fertilization itself. On the other hand, it means an act by 

 w hich the individuals become united w^hich belong to the two 

 sexes, i.e. a pairing, and this is the reason why we speak quite 

 generally of organs of copulation: they play the main role in 

 such a union. The etymology of the word itself shows that it 

 was originally coined for the second interpretation. It would 

 therefore be better if we use the equally w^ell-known word 

 syngamy when we speak of the union of the two gametes. 



It has already been mentioned that conjugation is considered 

 as an opposite to syngamy as if it were something entirely 

 different from the latter. This has been the main reason why 

 the Ciliata— as the only Protozoa that have adopted con- 



