Software tends to change itself according to user requirements changes. In the object-oriented technology, it is important to find a way how classes evolve to cope with new user requirements or to obtain reusability. We propose that developers need to know the class evolution processes and to forecast their future evolution according to the system growth. Class specifications and class structures change inevitably, and classes may also be replaced during the class evolution processes. In this paper, we classify classes into three categories: the boundary species, the domain species and the common species, and discuss their evolution processes.