We give a simple argument for the exclusive existence of mirror and electromaghetic ion cyclotron modes in anisotropic high-$beta$ plasmas. It is shown that, in addition to a large domain of coexistence of both modes, two domains exist in parameter space $(A,beta_perp)$ where solely either mirror modes or electromagnetic ion cyclotron modes can be excited. In the overlap region the modes with the larger growth rate should win. However nonlinear effects may modify such a conclusion.
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