It was the rich tropical and sub-tropical vegetation in the botanical gardens in Palermo that helped Goethe to his decisive observations. The peculiar nature of the warmer regions of the earth enables the spirit to reveal itself more intensively than is possible in the temperate zone. Thus in tropical vegetation many things come before the eye which otherwise remain undisclosed, and then can be detected only through an effort of active thought. From this point of view, tropical vegetation is 'abnormal' in the same sense as was the proliferated rose which confirmed for Goethe's physical perception that inner law of plant-growth which had already become clear to his mind.