  In order to reveal the long-term evolution of relativistic jets in active
galactic nuclei (AGNs), we examine the dynamical evolution of variously-sized
radio galaxies [i.e., compact symmetric objects (CSOs), medium-size symmetric
objects (MSOs), Fanaroff-Riley type II radio galaxies (FRIIs)]. By comparing
the observed relation between the hot spot size and the linear size of radio
source with a coevolution model of hot spot and cocoon, we find that the
advance speed of hot spots and lobes inevitably show the deceleration phase
(CSO-MSO phase) and the acceleration phase (MSO-FRII phase). The deceleration
is caused by the growth of the cross-sectional area of the cocoon head.
Moreover, by comparing the hot spot speed with the sound speed of the ambient
medium, we predict that only CSOs whose initial advance speed is higher than
0.3-0.5c can evolve into FRIIs.
