[1] D.J. Watts and S.H. Strogatz, Nature (London) 393, 440 (1998).

[2] A.-L. Barab´asi and R. Albert, Science 286, 509 (1999).

[3] B. Bollob´as, Random Graphs (Academic Press, London, 1985).

[4] D.J. Watts, Small Worlds (Princeton University Press, Princeton, 1999).

[5] S.H. Strogatz, Nature (London) 410, 268 (2001); R. Albert and A.-L. Barab´asi, Rev. Mod. Phys. 74, 47 (2002).

[6] There are recent papers concerning the network aspect of language. For instance, there are networks defined by the co-occurrence of words within sentences [R.F.I. Cancho and R.V. Sole, Proc. Royal Soc. London B 268, 2261 (2001); S.N. Dorogovtsev and J.F.F. Mendes, Proc. Royal Soc. London B 268, 2603 (2001)] and the network of nouns [M. Sigman and G.A. Cecchi, Proc. Natl. Acad. Sci. U.S.A. 99, 1742 (2002)]. These networks are different from the conceptual network that we have conceived and studied.

[7] ftp://ibiblio.org/pub/docs/books/gutenberg/etext02/ mthes10.zip.

[8] A.-L. Barab´asi, R. Albert, and H. Jeong, Physica A 272, 173 (1999); ibid 281, 69 (2000).

[9] Z. Liu, Y.-C. Lai, N. Ye, and P. Dasgupta, "Connectivity distribution and attack tolerance of general networks with both preferential and random attachments," preprint (2002).

[10] This model can be better motivated in the context of social networks. In a friendship network, when a new person is introduced into the group, he or she has uniform probability to make a first friend. The probability of making new friends, however, will depend on this first connection, since a person is more likely to be introduced to the friends of his or her friends than to any other person.

[11] R. Albert, H. Jeong, and A.-L. Barab´asi, Nature (London) 406, 378 (2000).

[12] T. Nishikawa, A.E. Motter, Y.-C. Lai, and F. Hoppensteadt, "The smallest small-world network," preprint (2001).

[13] C. Koch and G. Laurent, Science 284, 96 (1999).

[14] M. Haverkort, L.A. Stowe, and B. Wijers, talk in: The Neurological Basis of Language Conference, Germany (2001).

[15] K.E. Stephan, C.C. Hilgetag, G.A.P.C. Burns, M.A. O'Neill, M.P. Young, and R. Kotter, Philos. Trans. R. Soc. London B 355, 111 (2000).

[16] V. Latora and M. Marchiori, Phys. Rev. Lett. 87, 198701 (2001).