From these two different types of connections it appears that the cita-tion network shows the existence of two different literature practices and of two different needs on the part of the scientist. (i) The research front builds on recent work, and the network be-comes very tight. To cope with this, the scientist (particularly, I presume, in physics and molecular biolohgy) needs an alerting service that will keep him posted, probably by citation indexing, on the work of his peers and colleagues. (ii) The random scattering of Fig. 6 corresponds to a drawing upon the tottality of previous work. In a sense, this is the portion of the network that treats each published item as if it were truly part of the eternal record of human knowledge. In subject fields that have been dominated by this second attitude, the traditional procedure has been to systematize the added knowl-edge from time to time in book form, topic by topic, or to make use of a system of classification optimistically considered ‘more or less eternal, as in taxonomy and chemistry. If such classi-fication holds over reasonably long pe-riods, one may have an okbjective means of reducing the world total of knowl-edge to fairly small parcels in which the items are found to be in one-to-one correspondence with some natural order.