  One initial and essential question of magnetism is whether the magnetic
properties of a material are governed by localized moments or itinerant
electrons. Here we expose the case for the weakly ferromagnetic system
FeGa$_{3-y}$Ge$_y$ wherein these two opposite models are reconciled, such that
the magnetic susceptibility is quantitatively explained by taking into account
the effects of spin-spin correlation. With the electron doping introduced by Ge
substitution, the diamagnetic insulating parent compound FeGa$_3$ becomes a
paramagnetic metal as early as at $ y=0.01 $, and turns into a weakly
ferromagnetic metal around the quantum critical point $ y=0.15 $. Within the
ferromagnetic regime of FeGa$_{3-y}$Ge$_y$, the magnetic properties are of a
weakly itinerant ferromagnetic nature, located in the intermediate regime
between the localized and the itinerant dominance. Our analysis implies a
potential universality for all itinerant-electron ferromagnets.
