Is it permissible to wash the penis with water after urinating and cleansing with stones or dirt for defecation in the wilderness, or must ritual cleansing (istinja') for the front and back private parts be of one type (water or stones)?

There is no objection, when purifying oneself (istinja'), to using water for one orifice and restricting oneself to a stone or what serves its purpose for the other orifice. The obligation for both is to cleanse the area until the impurity is completely removed. It is best for a Muslim to combine water and stone in their purification, as it is more thorough in cleanliness. However, it is permissible to restrict oneself to either of them, and if water is available, it is better.