Evolution, in this context, does not mean changes in a hydrostatic transmission over time. Rather, it is the evolution from a primitive and simplistic ideal depiction to a more detailed and practical Type 2 model. The Type 2 model more closely represents a real-world hydrostatic transmission. This evolution is depicted in Fig. 1. Each stage of evolution has its place in hydraulic design and analysis.
Figure 1(a) is the simplest depiction, which shows both the pump and the motor in their ideal symbology. It explains the principles of the hydrostatic transmission. The pump symbol is on the left and has the counter-torque generator, which implies that loading effects at the pump are universal. The bidirectional pump-motor symbol suggests positive-displacement pumping action. The ideal motor uses the universal, bidirectional pump-motor symbol to characterize the flow-absorbing action of the motor displacement mechanism while the mechanical part of the circuit depicts the torque generation of the output motor.