The Romans adopted fighting styles and military reforms for each new enemy they encountered. They were the most progressive fighting force since Phillip II and Alexander (with the improved hoplite, sarissa spear and heavy cavalry reform that reinvigorated Macedonian supremacy) and as such left a similar mark. Their greek adopted hoplite warfare during the monarchy and early Republic didn't fare well in the mountainous middle regions of Italy and the Northern Alps, and as such Roman statesman Gaius Marius adopted the famous close flexible formations of swordsmen in small units capable of working independent of each other, partnered with javelins and a large degree of self-sufficiency. These units suited warfare in transalpine gaul, where they could be dispatched quickly and unhindered by large baggage trains or terrain to respond to the guerilla and unorthodox methods of gallic armies. Later into the late Republic and early empire, the cohort system was established which incorporated larger units and auxiliary forces from annexed and allied territories. Rome was no longer fighting gallic rabble, but organised gauls, large established kingdoms in the east and increasingly organised germanic tribes. These larger units were entirely self sufficient, with soldiers being able to to most if not all camp jobs and maintenance on their own without a baggage train or constant supply lines. Far from Rome these new legions worked wonders, however the increased independence led to loyalty to Generals rather than Rome, which led to many civil wars within the Republic/Empire as legions fought for the agenda of their General rather than exclusively the Senate/Emperor. Tl;dr the roman legions adapted to each new threat and it worked.

Sources: I'm on campus at the moment and will attach these when I get the time :)