History of Septic Systems

In the early days, when man wanted to have some privacy and protection from the elements when he did his "chores" he dug a hole in the ground, lined it with stone, brick, wood or other available material and built an "outhouse" above. Delivery of the waste to its final resting place was by gravity. If the hole eventually filled up, the outhouse got moved to a new location.
Upon Thomas Crapper's invention of the flush toilet, man was now able to do his chores in the comfort of his home. (See The Septic Information Website for link to Thomas Crapper)
Civilized man was now able to do away with the outhouse, but, because wastes and wastewater needed a place to go...
He connected the pipe to the pit that served the outhouse and covered over the hole to hold down the smell and keep the neighbor's dogs and kids from falling in.
The pit serving the toilet now was called a cesspool.
It soon became obvious that the cesspool often couldn't handle the extra load of the wastewater along with the waste.
Eventually somebody figured out that by putting a watertight tank in the line between the house and the cesspool, much of the waste could be removed from the flow of wastewater and trapped in the tank where it would decompose.
This treatment chamber became known as the septic tank.
Note that the septic tank has a baffle at each end to help keep waste in the tank. The original pit remained as the part of the system that returned "clarified" wastewater to the ground.
It now became known as a dry well.
Sometimes due to heavy use, poor soil conditions, age of the system or a combination of these factors, the drywell still plugged up. (Wastewater still contains soaps, greases and other stuff that seals the pores of all but the most porous soils.)
Often a second (or third or fourth) drywell would be installed after the first to increase the soil absorption area. Note that an alert installer would place a baffle at the outlet of the original drywell to help keep floating solids from passing into the new dry well.