33: 15-23. 76. Iwasa Y, F Michor, MA Nowak (2005). Virus evolution within patients increases pathogenicity.) theor Biol 232: 17-26. 77. Keenan EL, EP Stabler (2005). Linguistic invariants and language variation. In Logic, Methodology and Philosophy of Science, ed. P Hajck et al. London: Kings College Pub