One hardly always receives due credit for working out a general theory. It is more enticing to be successful in discovering new phenomena of tangible facts even if they are particular ones. The great technical opportunities which now exist and the new methods of investigation which recently seemed to be fantastic have made that even more enticing. Therefore, the establishment of general regu­ larities seems to be an abstract occupation whose significance is not always clear. Nevertheless, a general theory should be elaborated. Factual material should be systematized and generalized as it accumu­ lates. As an increasing amount of data is obtained, it becomes increasingly urgent to ascertain the general regularities which determine the typical mechanisms of individual phenomena. Re­ searchers approach these aspects of scientific activity in different ways. Some of them are interested in analyzing the specifics of a phenomenon, while others try to find a general regularity in an individual fact and look for its confirmation in new material. Both of these indispensable forms of research have been reflected in this book.