The Linux operating system embodies a development history of 15 years and community effort of hundreds of voluntary developers. We examine the structure and evolution of the Linux kernel by considering the source code of the kernel as ordinary text without any regard to its semantics. After selecting three functionally central modules to study, we identified code segments using local alignments of source code from a reduced set of file comparisons. The further stages of the analyses take advantage of these identified alignments. We build module-specific visualizations, or descendant graphs, to visualize the overall code migration between versions and files. More detailed view can be achieved with chain graphs which show the time evolution of alignments between selected files. The methods used here may also prove useful in studying large collections of legacy code, whose original maintainers are not available.