From this explanatory description, everyone will easily see the reason why - for natural reasons - this polar vitalizing center of gravity must not be fixed, but be variable; just as the blood in animals, as well as in man, represents something similar to this center of gravity. A fixed blood and even more so a fixed heart would certainly not serve any living being; in the animal bodies, however, which have free movement, the actual heart can already be given a more definite position, because the free movement of an animal body, as also that of man, already causes all kinds of reactions in itself, which, as is easily comprehensible, can certainly not be the case with those bodies which are not capable of any free movement. With these, the various necessary reactions must then be accomplished by the constantly changed position of the polar center of gravity. – The Earth, Chapter 3, Paragraph 8