I read this sentence in The Life of Samuel Johnson:
"The booksellers who contracted with Johnson, single and unaided, for the execution of a work, which in other countries has not been effected but by the co-operating exertions of many, were Mr. Robert Dodsley, Mr. Charles Hitch, Mr. Andrew Millar, the two Messieurs Longman, and the two Messieurs Knapton."
I am not certain about the meaning of "but" in this context. "But" means "only" OR "however". I can skip it and still understand the whole sentence. However, I want to know exactly about it because I am learning the English language.