Soldiers/Marines, mid-to-late Napoleonic era?

[derien]

Okay, you friends who know everything about the Master and Commander series, two questions (maybe it's only two):

1) In that time period, was a Marine always a Marine, or did they enlist in the Army and then sometimes get sent to sea?

2) If you don't know, who should I ask?

(And my tag is absolutely not a promise that I'm going to write anything.)