We are obliged to say that the places of the novae, of the planetary and stellar nebulae, and of the Wolf-Rayets in the evolutionary process are not certainly known. If the Wolf-Rayet stars have developed from the planetaries, the planetaries from the novae, and the novae have resulted from the close approach or collision of two stars, or from the rushing of a dark or faint star through a resisting medium, then the novae, planetaries and Wolf-Rayets belong to a new and second generation: they were born under exceptional conditions. The velocities of the planetary nebulae seem to be an insuperable difficulty in the way of placing them between the irregular nebulae and the helium stars. The average radial velocity of 47 planetary nebulae is about 45 km. per second; and, if the motions of the planetaries are somewhat at random, their average velocities in space are twice as great, or 90 km. per second. This is fully seven times the average velocity of the helium stars, and the helium stars in general, therefore, could not have come from planetary nebulae. The radial velocities of only three Wolf-Rayet stars have been observed, and this number is too small to have statistical value, but the average for the three is several times as high as the average for the helium stars. We can not say, I think, that the velocities of any novae are certainly known. If the planetaries have been formed from novae, especially the novae which encountered the fiercest resistance, the high velocities are in a sense not surprising, for those stars which travel with abnormally high speeds are the ones whose chances for collisions with resisting media are best; and, further, the higher the speeds of collision the more violent the disturbance. This line of argument also leads to the conclusion that the novae, planetaries and Wolf-Rayets belong not in general before the helium stars, but to another generation of stars. They may, and I think will, develop into a small class of helium stars having special characteristics; for example, high velocities. KANT'S HYPOTHESIS