This photo comes from the Velodrome Buffalo, a 333 meter track in Paris that hosted some of the most significant bicycle races in the world from 1893 until its destruction during WWI. This shot, dated to 1908, captures four early pacing motorcycles lined up on the Parisian track possibly for an exhibition race.
Monstrous pacing motorcycles, like these four solo-rider models were being used in the late 1800's and early 1900's to pace bicycle races. The pacers, also known as stayers created a slipstream or draft in which the cyclist could efficiently cover the majority of the race distance at a high speed. Team strategy would then coordinate when the pacer would pull into the pit allowing the racer to sprint the final laps. Before the introduction gasoline engine powered cycles, these already popular paced cycle races featured tandem bicycles with upwards of a half dozen riders leading the star cyclist. As the internal combustion engine was developed in Europe, it was introduced as a substitute for the multi-rider tandem bicycles. Predictably, the sight, sound, and smell of these motorized pacing machines quickly captivated the large crowds at cycle races which demanded more exhibitions of the thrilling contraptions, simultaneously giving birth to the motorcycle and the sport of motorcycle racing.
America wouldn't see its first motorized pacing machine until the last days of the 19th century, a few years after they became popularized throughout Europe. When pacing motorcycles did make their debut in the States, like with the Orient tandem pacer for example, they were a good bit lighter than these European behemoths seen in the photo. Machines like these represent an important and early milestone in the development of the modern motorcycle and illustrate how far the beloved machine has come in the century since their firey introduction.