  By using a non-local and time-dependent convection theory, we have calculated
radial and low-degree non-radial oscillations for stellar evolutionary models
with $M=1.4$--3.0\,$\mathrm{M}_\odot$. The results of our study predict
theoretical instability strips for $\delta$ Scuti and $\gamma$ Doradus stars,
which overlap with each other. The strip of $\gamma$ Doradus is slightly redder
in colour than that of $\delta$ Scuti. We have paid great attention to the
excitation and stabilization mechanisms for these two types of oscillations,
and we conclude that radiative $\kappa$ mechanism plays a major role in the
excitation of warm $\delta$ Scuti and $\gamma$ Doradus stars, while the
coupling between convection and oscillations is responsible for excitation and
stabilization in cool stars. Generally speaking, turbulent pressure is an
excitation of oscillations, especially in cool $\delta$ Scuti and $\gamma$
Doradus stars and all cool Cepheid- and Mira-like stars. Turbulent thermal
convection, on the other hand, is a damping mechanism against oscillations that
actually plays the major role in giving rise to the red edge of the instability
strip. Our study shows that oscillations of $\delta$ Scuti and $\gamma$ Doradus
stars are both due to the combination of $\kappa$ mechanism and the coupling
between convection and oscillations, and they belong to the same class of
variables at the low-luminosity part of the Cepheid instability strip. Within
the $\delta$ Scuti--$\gamma$ Doradus instability strip, most of the pulsating
variables are very likely hybrids that are excited in both p and g modes.
