A theory of zero frequency and low growth rate (lower than the ion cyclotron frequency) modes in homogeneous plasmas with temperature anisotropies is presented. These modes are considered to propagate at large angles to the ambient magnetic field and are electromagnetic in nature. From the analysis of a general dispersion relation, with arbitrary electron to ion temperature ratio Te/Ti, it is found that the mirror (Te→0) and field swelling (Ti→0) modes are smoothly connected. Thus an anisotropic homogeneous plasma will always relax via one of these instabilities (as well as through the field-aligned ion cyclotron instabilities) for any value of Te/Ti, provided electron and/or ion temperature anisotropies exceed the required thresholds (determined in the Te = 0 and Ti = 0 limits). This broadens the applicability of this mechanism for producing low-frequency waves in space plasmas, since a cold species is no longer essential for plasmas with little or no spatial (density and/or temperature) gradients.