Carts were developed towards the end of the fourth millennium. At first, they were drawn by asses or oxen, which were preferable for transporting heavy loads. The two-wheeled model is called a cart. The merchandise carried in four-wheeled wagons was protected with a cover. This wagon was pulled by four animals attached to either side of the pole that projected from the main body. As a means of transportation, wagons were slow, going no more than 15 kilometers per day, and could be used only on fairly flat land. Because of these disadvantages, it was more profitable to transport goods on the backs of asses, since these animals could travel over uneven terrain and cover a distance of 20 kilometers in a day. However, their loads had to be distributed in bags on either side of their spine, since the saddle pack for carrying goods directly on an animal’s back was not invented until a little before our era.