Animal and machine
The body of an animal may well be compared with some machine like locomotive engine. Indeed the animal body is a machine. It is a machine composed of many parts, each part doing some particular kind of work for which particular kind of structure fits it, and all the parts are dependent on each other and work together for the accomplishment of total business of the machine. The locomotive must be provided with fuel, such as coal, or wood, or other combustible substance. The consumption of which furnishes the force or energy of the machine. The animal body must be provided with fuel which is called food, which furnishes similarly the energy of the animal. Oxygen must be provided for the combustion of fuel in the locomotive and the food of the body. The locomotive is composed of special parts: the fire box for reception and combustion of fuel, the steam pipe for the carriage of steam, the wheels for locomotion, and the smokestack for throwing off waste. The animal body is similarly composed of parts: alimentary canals for reception and assimilation of food, excretory organs for throwing off the waste matter, the arteries and veins for the carriage of oxygen and food-holding blood, the legs and wings for locomotion.
