"It was not until the Middle Ages that the letter ⟨W⟩ (originally a ligature of two ⟨V⟩s) was added to the Latin alphabet, to represent sounds from the Germanic languages which did not exist in medieval Latin, and only after the Renaissance did the convention of treating ⟨I⟩ and ⟨U⟩ as vowels, and ⟨J⟩ and ⟨V⟩ as consonants, become established. Prior to that, the former had been merely allographs of the latter."
So at Julius Caesar's time, I and J were the same letter that you could write either way.
"It was not until the Middle Ages that the letter ⟨W⟩ (originally a ligature of two ⟨V⟩s) was added to the Latin alphabet, to represent sounds from the Germanic languages which did not exist in medieval Latin, and only after the Renaissance did the convention of treating ⟨I⟩ and ⟨U⟩ as vowels, and ⟨J⟩ and ⟨V⟩ as consonants, become established. Prior to that, the former had been merely allographs of the latter."
So at Julius Caesar's time, I and J were the same letter that you could write either way.