Identify how linguistic is related with historical linguistics?

Well, linguistics is the scholarly discipline whose subject matter is language.

Historical linguistics is the scholarly discipline whose subject matter is the development of language through time. It explains language in terms of how it historically developed to get to this point (its diachrony).

Up until the 1920s, historical linguistics was the mainstream of linguistics. Saussure, himself an important historical linguist, identified that language is a system which needs to be made sense of on its own terms, as a contemporary phenomenon (its synchrony).

This led to Structuralism, which uncovered a lot of the structures of language that Historical Linguistics had missed, because they were treating language as a process and not a system. Historical Linguistics for example did not really differentiate phones and phonemes; it didn’t need to. The distinction is essential to phonology. And after Structuralism, other approaches to linguistics have continued to be synchronic.

Historical linguistics has been marginalised in all of this, and is very much niche now. Historical linguistics has been enriched by our better understanding of synchronic linguistics, sociolinguistics, and pragmatics. But it is unfortunately out of fashion, with a lot of shortsighted linguists thinking it’s boring and old hat.

Fashion. Never forget that all of scholarship—not just the squishy humanities, but the sciences as well—are about fashion. There are fields of inquiry that fall out of fashion, and it’s not always for objectively sound reasons.