Scholars and students have always been great travelers. The official case for 'academic mobility' is now often stated in impressive terms as a fundamental necessity for economic and social progress in the world, and debated in the corridors of Europe but it is certainly nothing new. Serious students were always ready to go abroad in search of the most stimulating teachers and the most famous academies in search of the purest philosophy, the most effective medicine, the likeliest road to gold. Mobility of this kind meant also mobility of ideas, their transference across frontiers, and their simultaneous impact upon many groups of people. The point of learning is to share it, whether with students or with colleagues. One presumes that only eccentrics have no interest in being credited with a startling discovery, or a new technique. It must also have bean reassuring to know that other people in other parts of the world were about to make the same discovery or were thinking along the same lines, and that one was not quite alone, confronted by inquisition, ridicule or neglect. In the twentieth century, and particularly in the last 20 years, the old footpaths of the wandering scholars have become vast highways. The vehicle which has made this possible has of course been the aeroplane, making contact between scholars even in the most distant places immediately feasible, and providing for the very rapid transmission of knowledge. Apart from the vehicle itself, it is fairly easy to identify the main factors which have brought about the recent explosion in academic movement. Some of these are purely quantitative and require no further mention: there are far more centers of learning, and a far greater number of scholars and students. In addition one must recognize the very considerable multiplication of disciplines, particularly in the sciences, which by widening the total area of advanced studies has produced an enormous number of specialists whose particular interests are precisely defined. These people would work in some isolation if they were not able to keep in touch with similar isolated groups in other countries. Frequently these specializations lay in areas where very rapid developments are taking place, and also where the research needed for developments is extremely costly and takes a long time. It is precisely in these areas that the advantages of collaboration and sharing of expertise appear most evident. Associated with this is the growth of specialist periodicals, which enable scholars to become aware of what is happening in different centers of research and to meet each other in conferences and symposia(座谈会). From these meetings come the personal relationships which are at the bottom of almost all formalized schemes of cooperation, and provide them with their most satisfactory stimulus. According to the passage, scholars and students are great travelers because ______.