A comparison of 41 rotation curves suggests that grand design galaxies tend to have falling rotation curves between 0.8R_25_ and 1.2R_25_, while flocculent and multiple arm galaxies have nearly flat or rising rotation curves in this range. This observation is consistent with the theoretical prediction that long spiral arms require a high fraction of the total galaxy mass to be in the form of a cool disk population. The correlation also implies that spiral morphology is a semipermanent feature of a galaxy disk, like the rotation curve. In that case, isolated nonbarred flocculent galaxies probably cannot amplify m = 2 wavemodes from internal perturbations, so these galaxies are usually flocculent, whereas isolated nonbarred grand design galaxies can amplify such modes, so they are usually grand design. Interacting grand design galaxies with falling rotation curves, such as M51 and M81, may have had long spiral arms or global modes before their interactions, because there was always a strong amplifier for internal perturbations. The current spirals in these galaxies differ only because they result from the amplification of recent external perturbations.