348 THE EVOLUTION OF THE METAZOA 



micro- and macro-evolutions. It is my opinion that the 

 evolution of new types does not take place in one way only, 

 and that the rapidity of these changes can be widely different, 

 i.e. that there are species which are very old and conservative, 

 and species which are very young, of which we can say they 

 have just been developed. 



The fortunate discoveries from the younger geological ages 

 (e.g. that of the Archaeopteryx) can help us to get a clearer 

 idea how the development of a new sub-type, let us 

 say of a class, must be understood. De Beer (1954) has 

 shown how in this case and probably also in many other 

 cases within the frame of the Vertebrata, new^ mosaic-like 

 combinations of characteristics that had existed individually 

 and scattered within the frame of an older subtype (the 

 peculiarities of a type, however, in our case those of the 

 Vertebrata, are preserved) emerge as mutations parallel to the 

 changes of the environment. They develop into new combina- 

 tions and as such they become "new" peculiarities which 

 win a predominant role due to the working of natural 

 selection. 



This type of formation of new animal forms which should 

 be considered as representatives of higher taxonomic units 

 is now known under the name of Watson's rule. It has cer- 

 tainly not been the only element which influenced evolution. 

 It can be considererd also in connection with the evolution 

 of other animal types and, so it seems to me, also with 

 the evolution of the Cnidaria within the frame work of 

 the turbellarian type. Later we will return to discuss this 

 problem. 



There can be no doubt that the descendants which show 

 considerable variety and which therefore represent new 

 animal groups can be developed by w^ay of neoteny. The best- 

 known case which has also been studiedby way of experiments 

 is th2it oi Proteus ; it could be established that in a not very 

 remote past, a new subtype of tailed Amphibia had developed 

 by way of neoteny; this subtype was given the rank of a family 



