The importance of deviation-amplifying processes for the emergence of major evolutionary novelties is discussed by exemplifying the evolution of birds and the term ‘chain evolution’ is proposed.It is suggested that the importance of deviation-amplifying networks for the evolution of major systematic groups indicates that the changes leading to the origin of these groups progressed within a single genetic pool. The probability of polyphyletic origin of such taxonomic units as Tetrapoda, or Mammalia is regarded as extremely low.The diversity of the structure (...) of the central nervous system in different vertebrate groups is explained as resulting from multiple connections between it and the biology of the group. These connections form networks which may act either in a stabilizing or a deviation-amplifying way. Some examples of the networks are briefly discussed. (shrink)