Xenophanes for the first time, asserted that the fossil impressions of animals and plants were real remains of formerly living creatures, and that the mountains in whose rocks they were found must at an earlier date have stood under water. But although other great philosophers of antiquity, and among them Aristotle, also possessed this true knowledge, yet throughout the illiterate Middle Ages, and even with some naturalists of the last century, the idea prevailed that petrifactions were so-called freaks of nature (lusus naturæ), or products of an unknown formative power or instinct of nature (nisus formativus, vis plastica). Respecting the nature of this mysterious and mystic creative power, the strangest ideas were formed. Some believed that this constructive power—the same to which they also ascribed the coming into existence of the present species of animals and plants—had made numerous attempts to create organisms of different forms, but that these attempts had only partially succeeded, had often failed, and that petrifactions were nothing more than such unsuccessful attempts. According to others, petrifactions originated from the influence of the stars upon the interior of the earth. Others, again, had the still cruder notion that the Creator had first made models (out of mineral substances—for example, of gypsum or clay) of those forms of animals and plants which he afterwards executed in organic substances, and into which he breathed his living breath; petrifactions were accordingly such rude inorganic models. Even as late as the last century these crude ideas prevailed, and it was assumed, for example, that there existed a special “seminal air,” which was said to penetrate into the earth with the water, and by fructifying the stones formed petrifactions or “stony flesh” (caro fossilis). It took a very long time before the simple and natural view was accepted, namely, that petrifactions are in reality nothing but what they appear to simple observation—the indestructible remains of extinct organisms. It is true the celebrated painter, Leonardo da Vinci, in the 15th century, ventured to assert that the mud which was constantly deposited by water was the cause of petrifactions, as it surrounded the indestructible shells of mussels and snails which lay at the bottom of the waters, and gradually turned them into solid stone. The same idea was maintained in the 16th century by a Parisian potter, Palissy by name, who became celebrated on account of his invention of china. However, the so-called “professional men” were very far from paying any regard to these correct assertions of a simple and healthy human understanding; it was not till the end of the last century that it was generally accepted, in consequence of the foundation of the Neptunian geology by Werner.