It is thought by some that the need of vessels which would endure the heat was the cause of the invention of pottery. While there seems to be little evidence of this, it is easy to conjecture that when water was needed to be heated in a basket, a mass of clay would be put on the bottom of the basket before it was put over the coals of fire. After the cooking was done, the basket could easily be detached from the clay, leaving a hard-baked bowl. This led to the suggestion of making bowls of clay and baking them for common use. Others suggest that the fact of making holes in the ground for cooking purposes gave the suggestion that by the use of clay a portable vessel might be made for similar purposes. {92} The economic value of cooking rests in the fact that a larger utility comes from the cooked than from the raw food. Though the phenomena of physical development of tribes and nations cannot be explained by the chemical constituents of food, although they are not without a positive influence. Evidently the preparation of food has much to do with man's progress, and the art of cooking was a great step in advance. The better utilization of food was a time-saving process--and, indeed, in many instances may have been a life-saving affair. _The Domestication of Animals_.--The time and place of the domestication of animals are not satisfactorily determined. We know that Paleolithic man had domesticated the dog, and probably for centuries this was the only animal domesticated; but it is known that low forest tribes have tamed monkeys and parrots for pets, and savage tribes frequently have a band of dogs for hunting game or guarding the hut. While it may be supposed that domestication of animals may have occurred in the prehistoric period, the use of such animals has been in the historic period. There are many evidences of the domesticated dog at the beginning of the Neolithic period. However, these animals may have still been nearly half wild. It is not until the period of the Lake Dwellings of Switzerland that we can discriminate between the wild animals and those that have been tamed. In the Lake Dwelling debris are found the bones of the wild bull, or _urus_, of Europe. Probably this large, long-horned animal was then in a wild state, and had been hunted for food. Alongside of these remains are those of a small, short-horned animal, supposed to have been domesticated. Later, though still in the Neolithic period, remains of short-horned tame cattle appear in the refuse of the Lake Dwellings. It is thought by some that these two varieties--the long-horned _urus_ and the short-horned domesticated animal brought from the south--were crossed, which gave rise to the origin of the present stock of modern cattle in central Europe. Pigs and sheep were probably domesticated in Asia {93} and brought into Europe during the later Neolithic or early Bronze period.