The Secretary of the Interior is authorized, in his discretion, to accept, as hereinafter provided, on behalf of the United States, title to the lands referred to in section 404 of this title, and to be purchased with the funds which may be subscribed by or through the Mammoth Cave National Park Association of Kentucky, and with other contributions for the purchase of lands in the Mammoth Cave National Park area: Provided, That any of said lands may be donated directly to the United States and conveyed to it, cost free, by fee-simple title, in cases where such donations may be made without the necessity of purchase.