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Yet his scheme, with all its deficiencies, bore the authentic stamp of genius—of genius imperfectly equipped with knowledge, but original, penetrative, divinatory. The very entitling of the work, A Natural History of the Heavens was an audacity implying a radical change of conception. It was in this remarkable treatise that 'island universes' made their definitive appearance. Wright, it is true, had, five years previously (in 1750), thrown out the idea that 'cloudy spots' might represent 'external creations,' but as a mere vagary of the scientific imagination. Kant unhesitatingly laid hold of it, classed nebulæ as so many separate galaxies, and regarded them as combining with our own into a revolving system on a surpassing scale of grandeur. Kant was also the first to take into account the effects on their development of the plasticity of the heavenly bodies. He published in 1754, in a Königsberg paper, by way of preliminary to his forthcoming Natural History, an outline of the workings of tidal friction in the earth-moon system. He saw clearly that it had acted in the past to reduce our satellite's rotation to its present minimum rate, and that it even now, by very slow degrees, tended to retard the spinning of the earth. This brilliant forecast remained unnoticed for well-nigh a century.