make the following text more cohesive:
In these three volumes, which appeared at long intervals, the author's theories are not always in complete harmony. They theories not always presented in a very luminous or coherent manner, theories they are marked by originality and vigour.
In these three volumes, which appeared at long intervals, the author's theories are not always in complete harmony, nor are they always presented in a very luminous or coherent manner, but they are marked by originality and vigour.