The original Martini did not start off with these ingredients or in these proportions. The original Martini was made of 2 oz Old Tom gin, 1 oz dry vermouth and two dashes of orange bitters. That became know as the Sweet Martini. Slightly later the Dry Martini came about that used London Dry Gin instead but retained the original proportions.
Even today the Martini has several variations when it comes to how much dry vermouth is added. Google it for more info but personally I’m a fan of the Churchill style which is just chilled gin with no vermouth added at all.