The original John Collins used genever liquor. Later when Old Tom gin gained popularity in the mid to late 1800’s they changed the name to Tom Collins and used Old Tom gin instead. Old Tom gin stopped being produced in the 1930’s and around the early 1900s London dry gin became the popular gin of choice.
Just to confuse us all people started calling this drink a John Collins again. So, there are two versions of the John Collins- using either genever or London dry gin.