Third, Interlanguage is dynamic and it is a system of flexibility and variation. After entering the interlanguage system, new language rules have a strong ability to spread, so the system is under the constant restruction. Compared with some other forms of language growth, interlanguage are typically changing rather fast in developmental terms as learners receive more input and revise their hypotheses about L2.源]自[751^`论\文"网·www.751com.cn/

     Last but not least, interlanguage may be fossilized. Fossilization is the relatively stable embodiment of interlanguage. The most striking characteristic of interlanguage lies in that it is fossilizable, which tends to happen in various situations.

4. Fossilization and Its Classification

     The original definition by Selinker in 1972 is the most widely accepted one, which goes like the following:“Fossilization linguistic phenomena are linguistic items，rules and subsystems which speakers of a particular native language will tend to keep in their interlanguage relative to a particular target language，no matter what the age of the learner or the amount of explanation and instruction he receives in the target language” .In a word, fossilization is a process occurring from time to time in which incorrect linguistic features become a permanent part of the way a person speaks or writes a language.

4.1 Inpidual Fossilization and Group Fossilization

     As its name implies, inpidual fossilization refers to different fossilized items or different degrees of fossilized language competence that occur in different inpiduals. It is reflected in two aspects. The first is error appearance, which refers to the repeated appearance of errors that have been corrected many times but still remain in interlanguage. The second is language competence fossilization, which refers to the fossilization of the pronunciation, structure and vocabulary of the interlanguage. Error fossilization can be easily found in the beginners or learners with low proficiency. On the contrary, language competence fossilization is always found in the L2 learners who have been learning the TL for a long period of time or the learners who are very fluent speakers of L2.

     Group fossilization comes into being when fossilization occurs to a whole language community instead of different inpiduals, or even to a larger society such as a nation. It may lead to a new dialect like Indian English and Singapore English. The result of the study on group fossilization always throws light on inpidual fossilization.