green and red symbols represent the time evolution of the spiral arms for the two models that are essentially identical (see footnote). We found that the simulated spiral arms exhibit a maximum amplitude around $i \sim 30^{\circ}$. Further, we found that the spiral arms tend to be weaker in the pure stellar disk model (model MS) than the model with the ISM (model MSG) [^5]. This distribution which has a peak at around $i \sim 20-30^\circ$ is qualitatively consistent with the the prediction of the swing amplification mechanism (see section 4.1). We compared the simulated distribution with the observed one. The dependence of $|A_{m}|$ on the pitch angle in the models with/without the ISM is consistent with observations at least for the range of values corresponding to $i < 30^{\circ}$. This comparison suggests that the self-induced spiral arms in differentially rotating disks and their time evolution can fairly consistently account for the morphological diversity seen in nearby spiral galaxies.

However, this comparison between the simulated and observed distributions requires further examination. Firstly, the determination of the pitch angle is somewhat uncertain. The analyzed pitch angles differ between studies for the same galaxy. For example, the pitch angles of NGC 3054 