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Published online by Cambridge University Press: 14 August 2015
It is, I believe, of fundamental importance to find out how far the structure and motions in the galaxies can be explained by gravitational theory. This does not deny that hydrodynamic and electromagnetic forces may play an important part. The special point which I have in mind is the possibility, from the standpoint of gravitational theory, for the spiral structure to be a kind of quasi-steady state of comparatively long lifetime.