ARlogo Annu. Rev. Astron. Astrophys. 2004. 42: 603-683
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3.5. Conclusion

Barred galaxies give us a rich picture of secular evolution at work. One robust consequence is the buildup of the central mass concentration via the inward radial transport of gas. Infrared imaging shows that the majority of spiral galaxies have bars. Theoretical arguments and observational evidence suggest that similar processes are at work in many unbarred galaxies, especially in oval galaxies and in late-type, global-pattern spirals. In late-type galaxies, it is relatively easy for the central mass concentration that we see to be caused by secular processes, because the evolution happens most readily if a galaxy does not already have a classical bulge.